We present an improved differential phase shift quantum key distribution schemewhich features auto-compensation，high efficiency，and excellent practicality. Alice modulates continuous laser using an intensity modulator based on polarization to generate coherent pulse train. Bob employs a Fraday-Michelson interferometer instead of a traditional M-Z interferometer，which automatically compensates for the polarization mode dispersion caused by the changes of environment and improves the stability of the interference visibility. The present scheme not only has a simpler configuration，but also offers a higher key creation efficiency than traditional schemes. Furthermore，it enhances the security of the system. With the proposed experimental setup，a stable quantum key distribution was performed over 80km fiber with a quantum bit-error rate of less than 4%.