A synchronization method, an apparatus, and a system, which relate to the communications field and applied to synchronization signal transmission to implement synchronization of data frame transmission between devices on an unlicensed carrier are provided. The synchronization method is applied to synchronization on an unlicensed carrier. A network device sets a synchronization signal in a first subframe, and the network device sends the first subframe or the first subframe and a second subframe to user equipment, where the first subframe includes M orthogonal frequency division multiplexing (OFDM) symbols, the second subframe includes N OFDM symbols, M and N are positive integers, M>N, and the first subframe and the second subframe are subframes of an unlicensed carrier.