Abstract: In order to improve the performance of a servo system for background projection system, its multi mode control algorithm is modified. A different tri mode control algorithm is adopted according to different angles of rotation of the system. That is, a tri mode rule which includes the saturation control, the square root control, the PID control+feed forward control and certain position offset is used for large angles. When the angle is small, the saturation control is replaced by the reverse square root control. Using this algorithm, the time spent for the system on rotating from -90° to 90°, which used to be 0 6 s, has been shortened to 0 2 s, and the maximum position error of the system made less than 0 5 mrad at the premise of non overshoot and no vibration.