The embodiments of the present invention are based on a wireless local area network (WLAN) technology. An application scenario of the embodiments of the present invention is as follows: Before communicating with a STA, an AP needs to establish an association with the STA, and an important step of establishing an association is that the STA needs to obtain an AID allocated by the AP to the STA, where the AID is used to uniquely identify the STA. Usually, the AP receives association request messages sent by multiple STAs. The multiple STAs may send the association request messages based on an OFDMA technology. The AP may send an association acknowledgement message to the multiple STAs in two manners. A first manner is an OFDMA acknowledgement. That is, the AP sends an acknowledgement ACK or a block acknowledgement BA to the multiple stations on orthogonal subchannels. The ACK is an acknowledgement of a single Media Access Control Protocol data unit (MPDU). The BA is an acknowledgement of an aggregate-Media Access Control Protocol data unit (A-MPDU). A second manner is an M-BA frame acknowledgement, which is a broadcast message. That is, the AP sends, to the STAs by means of broadcasting by using an M-BA frame, acknowledgements to be sent to the multiple STAs. As shown in FIG. 1, the M-BA frame includes: a control frame, a duration/an identifier (duration/ID), a receive address (RA), a transmit address (TA), block acknowledgement control (BA control), block acknowledgement/acknowledgement information (BA/ACK Info), and a frame check sequence (FCS). The block acknowledgement control includes a block acknowledgement/acknowledgement (BA/ACK) policy, a multi-traffic identifier (Multi-TID), a compressed bitmap, groupcast with retries (GCR), a reserved field, and traffic identifier information (TID Info). The TID info is used to indicate how much BA/ACK Info exists. Further, the BA/ACK Info includes per traffic identifier information (Per TID Info). When the BA/ACK Info is BA Info, the BA/ACK Info further includes block acknowledgement starting sequence control and a block acknowledgement bitmap. Still further, an AID of a STA is set in the first 11 bits of the Per TID Info, and is used to indicate a station to which the AP is to send an acknowledgement. The 12^th bit is a block acknowledgement/acknowledgement indication (BA/ACK Indication). The 13th to the 16th bits are a traffic identifier TID. The following describes in detail a method for establishing an association between an AP and a STA.