Abstract: In order to obtain good time and frequency domain quality of a fast steering mirror (FSM), a method based on improving root locus was used to design its control parameters. At first, the root locus of FSM driven by a voice coil motor was established according to the identificated dynamic model. Then the expected dominant pole position was calculated based on the performance specification of the control system. By configuring the zero position of the controller, the root locus of the system was expected to have the dominant pole. Finally, the gain of system and the pole position of controller were obtained according to the modulus value formula and phase angle formula. The results show that the FSM control system based on the improved root locus algorithm has good dynamic performance and can suppress the mechanical resonance efficiently. The method is feasible and versatile.