The present invention discloses an automatic gain control method and a communications device in a wireless local area network. The method includes: generating a physical layer packet, where the physical layer packet includes a high efficiency long training field, the high efficiency long training field includes N symbols, a length of a cyclic prefix CP of a first symbol in the N symbols is greater than or equal to a minimum length required by a receiver device to perform automatic gain control AGC estimation, and N is a positive integer; and sending the physical layer packet to the receiver device. System overheads can be reduced according to embodiments of the present invention.