A new differential phase shift quantum key distribution system based on the intensity modulator with Faraday-Michelson interferometer is demonstrated experimentally. Some technologies are employed to improve the performance of the quantum key distribution system. These technologies include the clock generation technology of arbitrary frequency by a small scale frequency-split and phase locked loop scheme, the new method to acquire the half-wave voltage of the phase modulator with a high accuracy in the condition of the single photon count, and the auto-compensated polarization technology by the Faraday-Michelson interferometer. Finally, the quantum key distribution is achieved with a quantum bit error rate of 3.9% when the transmission distance reaches 50 km.