This application provides a signal transmission method, apparatus, and system, and a device. The method may include determining, by a second access network device, that a first channel is idle. The method may also include sending, by the second access network device, first information that indicates occupancy of the first channel, where the first information includes a first power value and first duration, and the first power value is power used by the second access network device to send a signal through the first channel in the first duration. Therefore, spectrum efficiency of an unlicensed spectrum is improved.