This application provides a method for transmitting a wake-up packet in a communications system and a device. The communications system includes a transmit end device and a receive end device. The receive end device includes a wake-up receiver and a main transceiver. The method includes: sending, by the transmit end device, a first wake-up packet WUP to the wake-up receiver by using a first data rate; determining, by the transmit end device, a second data rate; and sending, by the transmit end device, a second WUP to the wake-up receiver by using the second data rate. The transmit end device can determine a proper data rate for sending a wake-up packet based on a specific transmission condition in a communication process, thereby improving transmission efficiency of the wake-up packet.