As an important component of the composite-axis photoelectric tracking system, the upper limit of the tracking accuracy of the system is determined by the perturbation suppression ability and dynamic response ability of the fast steering mirror itself. In order to improve the performance of the fast steering mirror system, a fractional-order active disturbance rejection controller based on the existing active disturbance rejection control theory and combined with the fractional-order theory is proposed. The design process of the controller is given, and the control effect of two control strategies to the fast steering mirror performance, i.e. the traditional active disturbance rejection controller (ADRC) and the fractional-order active disturbance rejection controller composed of the expanded state observer and fractional-order controller, is compared and analyzed through simulation and experimental verification. Experimental results show that the fractional-order active disturbance rejection controller improves the fast steering mirror rapidity by 20.58% compared with the traditional active disturbance rejection controller in the case of step response, while the fast steering mirror tracking accuracy improves by 26.9% in the case of sinusoidal tracking.