A light synchronization scheme to solve the key problem of signal synchronization in quantum key distribution (QKD) system is proposed. At side of the transmitter, the field-programmable gate array (FPGA) chip produces a synchronization pulse sequence of certain pattern. The pulse sequence drives laser to transmit sync light through one fiber along with signal light to the receiver. The sync light reverts to sync pulse sequence through fast discrimination at side of the receiver, and the pulses are provided to the single-photon detector as gate after high-precision delay adjustment. Simultaneously, the receiver uses a high-frequency clock to sample the sync pulse sequence in FPGA for encoding, synchronizing the transmitter and receiver. The scheme has a high-precision, low-cost, high fault tolerance feature, and was successfully applied to metro quantum communication network.