The reflection process of the fast-steering mirror in an electro-optical tracking system was researched to improve its tracking precision. Firstly, the relationship of the turning angles between fast-steering mirror and reflection light was deduced, and the control method of the fast-steering mirror system was expatiated in detail as well as the software realization. Then, by taking the relationship mentioned above for the thesis, a servo control system for the fast-steering mirror was established. Furthermore, zero-locked and tracking experiments were performed to verify the performance of the control system, and the experimental results for the fast-steering mirror system and the main shafting system were compared. Obtained results indicate that the fast-steering system has fast-response and high-precision, and the residual error is less than 1″ in the zero-locked experiment. Moreover, the Root Mean Square (RMS) error of azimuth tracking in the fast-steering system is 3.6″ and the RMS error of elevation tracking is 8.7″ in the tracking experiment, which meets the requirement of tracking system for the tracking speed and pointing precision. The conclusion is that the servo system for fast-steering mirror based on the thesis on reflection process is characterized by high precision and fast response.