We propose and demonstrate an improved differential phase shift key distribution scheme. Alice uses a pulsed laser source and two cascaded fiber loops to get coherent pulses, then modulates them randomly under differential phase shift scheme, which compensates for the random polarization mode dispersion and phase shift. An interferometer made up of two Faraday mirrors is employed to detect single photon under gate mode. In one-way transmission, QKD is free from Trojan attack. Our experiment at setup can work stably for a long time (at least 24h) with an error ratio of 3%. The stable scheme is efficient, economical and favorable for practical application.