Fast-steering mirror is widely used in various kinds of space optical fields which have fast response speed, high control bandwidth and high ability of anti-interference. Although there are some researches on FSM, there are few studies on the design of the control system. According to the working principle of FSM, the control algorithm reliability in engineering, the cost and size of the circuit and so on, this paper designs a FSM control system based on STM32F4 ARM, which adopts disturbance observer and PID closed-loop control algorithm. The experimental results show that compared with the traditional PID control algorithm, the disturbance observer and PID control algorithm have the characteristics of high dynamic stability and high robustness. The control system of FSM based on ARM is working well, which can finish the calculation of the control algorithm in real time, and is convenient for engineering.