Disclosed are a signal transmission method and apparatus. The method comprises: a network device acquiring an end moment of first downlink transmission, wherein the first downlink transmission is one or more downlink sub-frames, or a downlink transmission part in a self-contained sub-frame, or a combination of the one or more downlink sub-frames and the downlink transmission part in the self-contained sub-frame; starting from the end moment of the first downlink transmission, the network device carrying out second downlink transmission in an unauthorized frequency band after a first protection interval has elapsed; at least one terminal device carrying out channel interception in the first protection interval; and the second downlink transmission being used for reserving a channel for the at least one terminal device, so that the terminal device is prevented from sending any signal, saving on the transmission power of the terminal device.