Embodiments of this disclosure provide a signal transmission method and a communications device. The signal transmission method includes: obtaining auxiliary information of a synchronization signal of a preset operation frequency band; and transmitting or receiving the synchronization signal based on the auxiliary information, where the preset operation frequency band is an operation frequency band of a preset service, and the preset service is a sidelink service, an mMTC service, an NB-IoT service, or an IAB service; or the preset operation frequency band is an unlicensed frequency band.