The invention belongs to the technical field of quantum secret communication, and particularly discloses a synchronizing device for a quantum key distribution (QKD) system. The synchronizing device comprises a transmitting device for transmitting synchronizing light and signal light, a photoelectric conversion device, an adjustable time delay module, a single photon detector and a data processing module, and is characterized in that the frequency of the signal light sent by the transmitting device is much greater than the frequency of the synchronizing light, the frequency of the signal light is several times that of the synchronizing light, the synchronizing light and the signal light are transmitted in the same optical fiber, a low jitter frequency doubler is arranged between the photoelectric conversion device and the adjustable time delay module and is used for doubling the frequency of a synchronized electrical signal output by the photoelectric conversion device to be the same as the frequency of the signal light, and then outputting the signal, the adjustable time delay module uses the signal to generate a door control signal of the single photon detector with the same frequency of the signal light, and the data processing module uses the signal to finish subsequent processing. The synchronizing device can reduce the impact of the synchronizing light on the signal light, can guarantee normal door control signal of the detector, and can effectively improve the code rate of the QKD system.