Abstract: In free-space optical communication system, the resolution is a very important parameter, which affects the performance of the whole system. As a common detector in the sub-system of APT, PSD has some advantages, such as high resolution, no blind area, high-speed response, etc. The basic architecture of PSD and its detection theory are analyzed, the position resolution in spatial optical communication achieved with PSD is investigated, and the methods are presented to improve the position resolution of PSD. A set of experiment system was set up to analyze the resolution of PSD in sub-system of APT. Through the analysis of the data which we achieve, the average position resolution is up to 12.6μm which agrees with theoretical resolution quite well. The feasibility of PSD as a fine tracking detector in the sub-system of APT is proved.