To improve the optical axis alignment accuracy of airborne laser communication system under the disturbance of body vibration and mechanical friction, a back-stepping sliding mode control method based on sliding mode observer was proposed. Firstly, the mathematical model of the airborne laser communication system was established, and then the disturbance value was estimated by the designed sliding mode observer. At the same time, the back-stepping sliding mode control law was gradually designed for the command conversion module, laser communication module and motor module, which realized the high-precision control for the optical axis of the airborne laser communication system. The experimental results show that the proposed method has better rapidity and accuracy than the fractional PID control method, the response time is only 0.4 s, the maximum space alignment error is only 0.3 m, the designed sliding mode observer can estimate the disturbance value quickly and accurately, the response time is only 0.3 s, and the maximum estimation error is only 0.1 m/s, 0.06 (°)/s² and 0.07 A/s, which greatly improves the alignment accuracy of the optical axis in the airborne laser communication system.