Based on quick response performance and insensitivity to variation of plant parameters and external disturbance of the variable structure control,a variable structure controller with sliding mode (SMC) is designed for the separating-mirror servo system of infrared target simulator.The chattering of variable structure controller with sliding mode is reduced by using exponential reaching law with saturation function.Mathematical model of the separating-mirror is constructed by the mechanism analysis method.The application of S-function simplifies the simulation modeling of the complex system and improves the simulation speed.By the simulation research and comparison with simulation results of the PID controller,simulation result shows that,variable structure controller with sliding mode has a strong robustness for external disturbance and parameter changes of the separating-mirror,and it can guarantee simulation precision and fidelity of experiment results of infrared guidance simulation system.