Abstract: Fast-steering mirrors require a rapid dynamic response and an anti-interference ability. In this study, an improved sliding-mode controller based on an extended state observer is proposed to solve the uncertain interference caused by self-movement, external interference, and other factors in the working environment of a fast-steering-mirror system based on the analysis and mathematical modeling of the fast-steering-mirror system. The proposed system uses an extended state observer to observe the unknown disturbance and directly compensate it to the controller, which effectively reduces chattering and facilitates engineering implementation. The simulation results show that compared with the traditional sliding-mode controller, the improved sliding-mode controller based on the extended state observer decreases the rise and adjustment times by 50.4% and 39.1%, respectively, and increases the tracking accuracy by 30.5%. These values satisfy the working requirements of the fast mirror and improves the dynamic performance.