A method for replying with an acknowledgement frame, wherein the method comprises: sending, by an access point, AP, in a wireless local area network, WLAN, a scheduling frame to a plurality of stations, STAs, associated with the AP in the WLAN; receiving, by the AP, a plurality of uplink physical layer protocol data units, PPDUs, that are in response to the scheduling frame, wherein each of the plurality of uplink PPDUs comprises a first preamble and an aggregate media access control protocol data unit, A-MPDU; calculating, by the AP, sending duration of a physical layer service data unit PSDU in a to-be-sent downlink PPDU based on a format and a sending rate that are of the downlink PPDU and based on that a length of each block acknowledgement bitmap field in a block acknowledgement frame in the downlink PPDU is a maximum quantity of bytes that is of each respective block acknowledgement bitmap field of the plurality of STAs and that is determined by the AP and the plurality of STAs in a session association phase, to generate a legacy signal, L-SIG, field of a second preamble, wherein the downlink PPDU comprises the second preamble and the PSDU, and the PSDU is the block acknowledgement frame for A-MPDUs in all the uplink PPDUs; generating, by the AP, the block acknowledgement frame, wherein the block acknowledgement frame comprises a block acknowledgement information field, the block acknowledgement information field comprises a plurality of STA information subfields, the plurality of STA information subfields have a one-to-one correspondence with the plurality of STAs, each STA information subfield comprises an association identifier traffic identifier, AID TID, field, a block acknowledgement starting sequence control field, and a block acknowledgement bitmap field, and a length of the block acknowledgement bitmap field in each STA information subfield is always equal to a maximum quantity of bytes of a block acknowledgement bitmap field that is determined by the AP and a corresponding STA in the session association phase; and sending, by the AP, the downlink PPDU; and wherein said calculating step is based on that the length of each block acknowledgement bitmap field in the block acknowledgement frame in the downlink PPDU is always fixed and is unrelated to whether the AP successfully receives the A-MPDU in the respective uplink PPDU. sending, by an access point, AP, in a wireless local area network, WLAN, a scheduling frame to a plurality of stations, STAs, associated with the AP in the WLAN; receiving, by the AP, a plurality of uplink physical layer protocol data units, PPDUs, that are in response to the scheduling frame, wherein each of the plurality of uplink PPDUs comprises a first preamble and an aggregate media access control protocol data unit, A-MPDU; calculating, by the AP, sending duration of a physical layer service data unit PSDU in a to-be-sent downlink PPDU based on a format and a sending rate that are of the downlink PPDU and based on that a length of each block acknowledgement bitmap field in a block acknowledgement frame in the downlink PPDU is a maximum quantity of bytes that is of each respective block acknowledgement bitmap field of the plurality of STAs and that is determined by the AP and the plurality of STAs in a session association phase, to generate a legacy signal, L-SIG, field of a second preamble, wherein the downlink PPDU comprises the second preamble and the PSDU, and the PSDU is the block acknowledgement frame for A-MPDUs in all the uplink PPDUs; generating, by the AP, the block acknowledgement frame, wherein the block acknowledgement frame comprises a block acknowledgement information field, the block acknowledgement information field comprises a plurality of STA information subfields, the plurality of STA information subfields have a one-to-one correspondence with the plurality of STAs, each STA information subfield comprises an association identifier traffic identifier, AID TID, field, a block acknowledgement starting sequence control field, and a block acknowledgement bitmap field, and a length of the block acknowledgement bitmap field in each STA information subfield is always equal to a maximum quantity of bytes of a block acknowledgement bitmap field that is determined by the AP and a corresponding STA in the session association phase; and sending, by the AP, the downlink PPDU; and wherein said calculating step is based on that the length of each block acknowledgement bitmap field in the block acknowledgement frame in the downlink PPDU is always fixed and is unrelated to whether the AP successfully receives the A-MPDU in the respective uplink PPDU.