The problem of mirror motion compensation was studied for a geostationary satellite to eliminate the optical axis direction deviation due to attitude deviation and normal line deviation of the mirror. The rotational dynamical equation of a spacecraft with two-degree freedom scan mirror was presented. The satellite attitude was described by Euler angles, and the normal line deviation of the mirror was described by Euler axis/angle parameters. The error transfer function from the two influencing factors to the optical axis direction was deduced. To resolve the problem of immeasurability of the normal line deviation, an estimation algorithm was presented using the precise direction finding ability of the scan mirror during special operation mode, and the long period character of the normal line deviation. The estimated value of the normal line deviation was replaced at set time intervals. Based on the estimation algorithm, a compensation algorithm was presented using the information of satellite attitude and normal line deviation, which compensated the scanning angle and the stepping angle separately. Numerical simulation results demonstrate the validity of the compensation algorithm.