Abstract: In the application of space target detection, due to the characteristics of small target and great distance, the returned signal is very weak, even a few of photons or less than a photon per pulse. Such weak signal can't be detected by traditional laser radar. Moreover, the space target detection now not only needs the range information, but hopes to get the intensity, polarization and more multi-dimensional information. Therefore, high sensitivity photon polarization laser radar system was proposed based on the four-way Gm-APD single photon detector. The intensity formula of weak signal was deduced from the Poisson probability model of Gm-APD detector. Then, the polarization information of the returned signal was obtained through four-way intensity results. Finally, the principle prototype of four-way Gm-APD photon polarization laser radar system was established in the laboratory, and the relevant experiment was conducted. When the average energy of returned signal per pulse is 7.7810-19 J, the experimental results show the linear polarization error of 3.95% and the elliptical polarization error of 5.84%.