A method for replying with an acknowledgement frame includes: calculating, by an access point (AP), sending duration of a physical layer service data unit (PSDU) in a to-be-sent downlink physical layer protocol data unit (downlink PPDU) based on a format and a sending rate that are of the downlink PPDU and based on only 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 stations (STAs), to generate an legacy signal field of a second preamble; and generating, by the AP, the block acknowledgement frame, where each STA information subfield in a block acknowledgement information field of the block acknowledgement frame includes an association identifier traffic identifier field, a block acknowledgement starting sequence control field, and a block acknowledgement bitmap field.