This paper presents a kind of robust sliding mode controller (SMC) based on disturbance observer. The overall system consists of two parts: one is the design of disturbance observer (DO), the other is the design of discrete SMC controller. By using DO technology, the external disturbances can be compensated and the SMC control law can be used successfully. The proposed controller is adopted in flight simulator servo system. Simulation and real-time experiment indicate good performance improvement and high robustness can both be gotten