Embodiments disclose 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. The method also includes sending the physical layer packet to the receiver device.