What is claimed is:1. A first network device comprising a processor and a memory coupled to the processor, the memory comprising instructions that, when executed by the processor, cause the first network device to:configure a first resource set group of a physical downlink control channel set for a terminal device;configure, for the terminal device, a second resource set group of a physical downlink control channel for retransmission,wherein the physical downlink control channel for retransmission is a physical downlink control channel used for retransmission of downlink data, which is indicated by a physical downlink control channel in at least one subset of the physical downlink control channel set;send, to the terminal device, information about the first resource set group and information about the second resource set group;send at least one physical downlink control channel in the at least one subset to the terminal device,wherein the at least one physical downlink control channel carries first control information, and the first control information comprises scheduling information of the downlink data;receive negative acknowledgement information from the terminal device; andsend, to the terminal device, at least one of the physical downlink control channel that is for retransmission and in the second resource set group,wherein the at least one of the physical downlink control channel for retransmission carries second control information comprising scheduling information of the downlink data associated with the at least one of the physical downlink control channel for retransmission,wherein the downlink data retransmitted by the at least one of the physical downlink control channel is sent by a second network device to the terminal device, andwherein the second network device is controlled by the first network device.2. The first network device according to claim 1, wherein the information about the second resource set group comprises:an association between the physical downlink control channel in the at least one subset and the second resource set group or between the at least one subset and the second resource set group.3. The first network device according to claim 1, wherein the memory further comprises instructions that, when executed by the processor, cause the network device to:for the physical downlink control channel in the at least one subset, notify the terminal device of configuration information of a physical uplink control channel and/or transmit power configuration information of the physical uplink control channel, wherein the physical uplink control channel is used to carry acknowledgement information of the downlink data indicated by the physical downlink control channel.4. A terminal device comprising a processor and a memory coupled to the processor, the memory comprising instructions that, when executed by the processor, cause the terminal device to:store information that is about a first resource set group of a physical downlink control channel set and sent by a first network device to the terminal device;store information that is about a second resource set group of a physical downlink control channel for retransmission and sent by the first network device to the terminal device;receive at least one physical downlink control channel according to the information that is about the first resource set group;acquire first control information carried on the at least one physical downlink control channel;receive downlink data according to scheduling information that is of the downlink data and comprised in the first control information, wherein the at least one physical downlink control channel is a physical downlink control channel in at least one subset of the physical downlink control channel set;detect, in the second resource set group, information that is about the second resource set group, wherein the physical downlink control channel used for retransmission of the downlink data;wherein the information that is about the second resource set group associates the physical downlink control channel in the at least one subset and the second resource set group, or the subset of the physical downlink control channel set and the second resource set group;before detecting, in the second resource set group, the physical downlink control channel used for retransmission of the downlink data, determine the second resource set group associated with retransmission of the downlink data, according to the at least one physical downlink control channel and the information about second resource set group;receive, according to the information about the second resource set group, at least one of the physical downlink control channel that is for retransmission and in the second resource set group in response to the acknowledgement information being negative acknowledgement information,wherein the physical downlink control channel for retransmission carries second control information; andreceive retransmitted downlink data according to scheduling information comprised in the second control information, wherein the scheduling information is scheduling information of the downlink data retransmitted on the physical downlink control channel.5. The terminal device according to claim 4, wherein the memory further comprises instructions that, when executed by the processor, cause the terminal device to:for the physical downlink control channel in the at least one subset, receive configuration information that is of a physical uplink control channel and notified by the first network device;after receiving the downlink data according to the scheduling information that is of the downlink data and comprised in the first control information, determine, according to the configuration information that is of the physical uplink control channel, resource information of the physical uplink control channel used by the downlink data; andsend acknowledgement information of the downlink data according to the resource information that is of the physical uplink control channel.