This application provides a synchronization signal sending method and a communications device, and provides a synchronization signal receiving method in V2X sidelink transmission communication. A complementary sidelink synchronization signal is sent in a time interval for sending synchronization signal burst sets, and is generated by using a sidelink synchronization identification of a first communications device. The complementary sidelink synchronization signal may be flexibly configured in a data frame following a synchronization signal frame, and may be sent together with sidelink control information or sidelink data. In addition, sending or skipping sending the complementary sidelink synchronization signal is optional. As a supplement to a sidelink synchronization signal, the complementary sidelink synchronization signal occupies a few resources and is sent at high frequency. Therefore, synchronization reliability can be improved when a vehicle moves fast.