Based on time-division multiplexing technology, a quantum key distribution (QKD) system based on BB84 protocol is designed by using a single photon detector (SPD). The delay fiber with an increase of 6 m in turn is added to the different polarization signal optical path at the receiving end, which makes the arrival time of different polarization signal light to the SPD vary by 30 ns in turn. On the premise of accurate synchronization of signal and synchronous light, the detecting gate of different polarization signal light is numbered at the receiving end. The corresponding polarized photons are detected by SPD at different numbering gates. Security analysis results show that the QKD system can resist intercept-resent attack by eavesdroppers and ensure the security and reliability of communications. The QKD system is successfully applied to the 520 m QKD experiment. The key rate is 4.00 kbps and bit error rate is 1.5%.