A synchronous communication method is provided. The method includes: obtaining, by a first terminal, first configuration information, where the first configuration information includes a first parameter; determining, by the first terminal according to a global navigation satellite system (GNSS) clock, a transmission timeslot corresponding to a current moment in a device-to-device (D2D) communications system; determining, by the first terminal according to the transmission timeslot and the first parameter, whether the current moment is a synchronization-information-sending moment; if the current moment is the synchronization-information-sending moment, sending, by the first terminal, sidelink synchronization information at the synchronization-information-sending moment, where the sidelink synchronization information carries indication information, the sidelink synchronization information is used to synchronize a second terminal with the first terminal, and the indication information is used to indicate, to the second terminal, that the first terminal is a synchronization source that uses the GNSS clock.