What is claimed is:1. A method for a single card dual standby user equipment (UE) to camp on a network, comprising:when a circuit switched (CS) domain of a UE camps on a second-generation mobile communications technology (2G)/third-generation mobile communications technology (3G) network and a packet switched (PS) domain camps on a long term evolution (LTE) network, executing, by the UE, a routing area update process of the PS domain of the 2G/3G network when the UE meets a preset first condition, so as to make the PS domain of the UE camp on the 2G/3G network; andafter the PS domain of the UE camps on the 2G/3G network, turning off an LTE radio frequency processor,after the turning off an LTE radio frequency processor, further comprising:when the UE needs to send or receive data, and a network side supports an idle mode signaling reduction (ISR) mechanism, turning on the LTE radio frequency processor to make the PS domain of the UE camp on the LTE network;when the UE meets a preset second condition, executing a data sending or receiving process in the LTE network; andwhen the UE meets a preset third condition, executing a data sending or receiving process in the PS domain of the 2G/3G network, wherein, that the UE meets a preset second condition further comprises:when the UE receives a Paging message through the LTE network but does not receive the Paging message through the PS domain of the 2G/3G network, and signal strength of the LTE network meets the data sending or receiving requirement, sending a service request message to the MME in the LTE network; and when the UE receives the Paging message through the PS domain of the 2G/3G network after receiving the Paging message through the LTE network, skipping responding to the Paging message that is received through the 2G/3G network; andwhen the UE receives the Paging message through the PS domain of the 2G/3G network but does not receive the Paging message through the LTE network, and the signal strength of the LTE network meets the data sending or receiving requirement, skipping responding to the Paging message that is received through the 2G/3G network; and when the UE receives the Paging message through the LTE network within a preset third period of time after receiving the Paging message through the PS domain of the 2G/3G network, sending a service request message to the MME in the LTE network.2. A method for a single card dual standby user equipment (UE) to camp on a network, comprising:when a circuit switched (CS) domain of a UE camps on a second-generation mobile communications technology (2G)/third-generation mobile communications technology (3G) network and a packet switched (PS) domain camps on a long term evolution (LTE) network, executing, by the UE, a routing area update process of the PS domain of the 2G/3G network when the UE meets a preset first condition, so as to make the PS domain of the UE camp on the 2G/3G network; andafter the PS domain of the UE camps on the 2G/3G network, turning off an LTE radio frequency processor,after the turning off an LTE radio frequency processor, further comprising:when the UE needs to send or receive data, and a network side supports an idle mode signaling reduction (ISR) mechanism, turning on the LTE radio frequency processor to make the PS domain of the UE camp on the LTE network;when the UE meets a preset second condition, executing a data sending or receiving process in the LTE network; andwhen the UE meets a preset third condition, executing a data sending or receiving process in the PS domain of the 2G/3G network,wherein, that the UE meets a preset third condition comprises:when the UE receives a Paging message through the LTE network but does not receive the Paging message through the PS domain of the 2G/3G network, and the signal strength of the LTE network does not meet the data sending or receiving requirement, skipping responding to the Paging message that is received through the LTE network; and when the UE receives the Paging message through the PS domain of the 2G/3G network after receiving the Paging message through the LTE network, sending a service request message to a serving general packet radio service support node (SGSN) in the 2G/3G network;when the UE receives the Paging message through the PS domain of the 2G/3G network but does not receive the Paging message through the LTE network, and the signal strength of the LTE network meets the data sending or receiving requirement, when the UE does not receive the Paging message through the LTE network within a preset third period of time after receiving the Paging message through the PS domain of the 2G/3G network, sending a service request message to the SGSN in the 2G/3G network; andwhen the UE receives the Paging message through the PS domain of the 2G/3G network but does not receive the Paging message through the LTE network, and the signal strength of the LTE network does not meet the data sending or receiving requirement, sending a service request message to the SGSN in the 2G/3G network.3. A user equipment (UE), comprising:a memory; anda first processor coupled to the memory and configured to:when a circuit switched (CS) domain of a UE camps on a second-generation mobile communications technology (2G)/third-generation mobile communications technology (3G) network and a packet switched (PS) domain camps on a long term evolution (LTE) network, execute a routing area update process of the PS domain of the 2G/3G network when the UE meets a preset first condition, so as to make the PS domain of the UE camp on the 2G/3G network; andafter the first processor makes the PS domain of the UE camp on the 2G/3G network, turn off an LTE radio frequency processor,after the first processor turns off the LTE radio frequency processor, turn on the LTE radio frequency processor when the UE needs to send or receive data and a network side supports an idle mode signaling reduction (ISR) mechanism, so as to make the PS domain of the UE camp on the LTE network; or when the UE needs to send or receive data and the network side does not support the ISR mechanism, turn on the LTE radio frequency processor;when the UE needs to send or receive data and the network side does not support the ISR mechanism, initiate a tracking area update (TAU) request to a mobility management entity (MME) in the LTE network after the first processor turns on a radio frequency processor, so as to make the PS domain of the UE camp on the LTE network;a second processor, configured to: after the PS domain of the UE camps on the LTE network, execute a data sending or receiving process in the LTE network when the UE meets a preset second condition; anda third processor, configured to: after the PS domain of the UE camps on the LTE network, execute a data sending or receiving process in the PS domain of the 2G/3G network when the UE meets a preset third condition.4. The user equipment according to claim 3, wherein,the preset first condition is that data traffic sent and/or received within a preset first period of time is smaller than or equal to a preset first value, or the preset first condition is that an average data at which data is sent and/or received within a preset second period of time is smaller than or equal to a preset second value; andthat the UE meets a preset first condition comprises:the data traffic sent and/or received by the UE within the preset first period of time is smaller than or equal to the preset first value, or at which data is sent and/or received the average data rate at which the UE sends and/or receives data within the preset second period of time is smaller than or equal to the preset second value.5. The user equipment according to claim 3, further comprising: a detacher;wherein, that the UE meets a preset second condition comprises:setting, by the first processor, a state of being attached to the PS domain of the 2G/3G network to be detached.6. The user equipment according to claim 3, further comprising: a first Paging message processor;wherein, that the UE meets the preset second condition comprises:before the second processor executes the data sending or receiving process in the LTE network, when the UE receives a Paging message through the LTE network but does not receive the Paging message through the PS domain of the 2G/3G network, and signal strength of the LTE network meets the data sending or receiving requirement, the first Paging message processor sends a service request message to the MME in the LTE network; when the UE receives the Paging message through the PS domain of the 2G/3G network after receiving the Paging message through the LTE network, the first Paging message processor skips responding to the Paging message that is received through the 2G/3G network; or, when the UE receives the Paging message through the PS domain of the 2G/3G network but does not receive the Paging message through the LTE network, and the signal strength of the LTE network meets the data sending or receiving requirement, the first Paging message processor sends a service request message to the MME in the LTE network when the UE receives the Paging message through the LTE network within a preset third period of time after receiving the Paging message through the PS domain of the 2G/3G network, so as to make the second processor execute the data sending or receiving process in the LTE network.7. The user equipment according to claim 3, further comprising: a second Paging message processor;wherein, that the UE meets the preset third condition comprises:before the third processor executes the data sending or receiving process in the PS domain of the 2G/3G network, when the UE receives the Paging message through the LTE network but does not receive the Paging message through the PS domain of the 2G/3G network, and the signal strength of the LTE network does not meet the data sending or receiving requirement, the second Paging message processor skips responding to the Paging message that is received through the LTE network; when the UE receives the Paging message through the PS domain of the 2G/3G network after receiving the Paging message through the LTE network, the second Paging message processor sends the service request message to a serving general packet radio service support node SGSN in the 2G/3G network; or, when the UE receives the Paging message through the PS domain of the 2G/3G network but does not receive the Paging message through the LTE network, and the signal strength of the LTE network meets the data sending or receiving requirement, the second Paging message processor sends the service request message to the SGSN in the 2G/3G network when the UE does not receive the Paging message through the LTE network within the preset third period of time after receiving the Paging message through the PS domain of the 2G/3G network; or, when the UE receives the Paging message through the PS domain of the 2G/3G network but does not receive the Paging message through the LTE network, and the signal strength of the LTE network does not meet the data sending or receiving requirement, the second Paging message processor sends the service request message to the SGSN in the 2G/3G network, so as to make the third processor execute the data sending or receiving process in the 2G/3G network.8. The user equipment according to claim 3, further comprising:after the second processor or the third processor completes the data sending or receiving process, when the data traffic sent and/or received by the UE within a preset fourth period of time is smaller than a preset third value, turn off the LTE radio frequency processor.