An experimental digital communication system of polarization division multiplexed quadrature phase shift keying(QPSK) signal transmission and coherent detection is successfully built.In the experiment,the 40 Gbit/s polarization division multiplexed QPSK signal is transmitted over 100 km in single-mode fiber.At the receiver,the optical signal is mixed with a local oscillator and down-converted,then processed by digital signal processing(DSP) algorithm off-line.Comparing the bit error rates and constellations of the received signal under different optical signal-to-noise ratio(OSNR) values before and after transmission over fiber,the experimental results show that detected the DSP algorithm can overcome the major channel damage to the signal,and effectively recover the high-speed coherent signal.