A simple method of ion beam figuring was successfully employed for processing rectangular off-axis aspheric mirror. Based on the material removal mechanism, the normal removal characteristic was analyzed. Beam diameter, peak removal rate and volume removal rate are the three factors to evaluate the robustness of removal function with respect to little disturbance of incidence angle. According to the results of analyses and experiments, the method of processing rectangular off-axis aspheric mirror by ion beam figuring was appropriately simplified. To validate the feasibility of this method, an off-axis aspheric ellipsoidal mirror was processed in our IBF system. With three iterations of 200 minutes, the surface error was reduced from 0.294λ RMS to 0.05λ RMS and the total convergence ratio was 10. The experimental results indicate that IBF has inimitable material removal characteristics, can effectively improve surface precision and realize the rapid processing of ultra-precision rectangular off-axis aspheric mirror.