To improve the reliability of satellite system,the observer-based actuator fault diagnosis problem for satellite attitude control system was investigated.Considering the uncertainty and disturbance of the satellite attitude control system,a robust adaptive sliding mode observer was proposed.A Lyapunov function was given as the judgment condition to stabilize the observer.Then,the proposed observer was used for actuator fault reconstruction.At last,the simulation of the observer in a satellite attitude control system with actuator fault illustrates the effectiveness of the proposed approach.