The quantum key distribution (QKD) system and single-photon detector are introduced. For the reduction of the dark counts of the single-photon detector,a synchronized laser system which consists of a synchronized laser of the transmitter and a laser discriminator of the receiver is developed. The method of developing the synchronized laser discriminator including photo electricity translator,preamplifier,voltage discriminator and level translator is shown detailedly. Test results show that the synchronized laser discriminator works stably by long time experiments. Error discrimination and fault discrimination can not be found. Now the synchronized laser discriminator has worked well on the QKD system. Transmitter and receiver are synchronized exactly. The dark counts of the single-photon detector are reduced effectively.