Fine tracking plays an important role in space laser communication, and the Fine Pointing Mirror(FSM) is a key component in fine tracking system. Based on the optical vector reflection law, the accurate optical characteristics of FSM is deduced, which serves as the theoretical basis for fine tracking study. A fine tracking control system based on the FSM accurate optical characteristic is introduced. For system regulation, two important improvements to the simplex method are obtained. Then a digital simulation of the fine tracking is given. So an accurate control of the FSM is accomplished and the accuracy of the fine tracking system is improved. The diffractive superresolution theory is introduced into the space laser communication. By using 3-zone phase pupil filter, the point spread function (PSF) of the optical system is modified, and the optical energy distribution at the focal plane is modified accordingly. Then the diffraction spot size is decreased, and the optical resolution is increased, so the accucy of the fine tracking system is improved.