To solve the problem of the geostationary orbit (GEO) satellite perturbed by flexible solar wing for solar array drive assembly (SADA). A semi-physical platform centered on the electric load simulator is designed to accurately simulate the solar wing reaction moment with variable load characteristics. A load simulator dynamic model is developed, and an extended state observer is created by estimating the internal parameter errors of the system with external perturbations as composite disturbances. The effect of observation errors and disturbances on the system is eliminated by using the sliding mode control, and the system control quantity is calculated; finally, the stability of the system is proved by using the Liapunov method. The platform's response speed, loading accuracy, and error are analyzed under different signals, and the results show that the algorithm can simulate the desired solar wing reaction moment more accurately.