Abstract: Mast aspheric optical elements fabricated using computer controlled optical surfacing (CCOS) technology have midfrequency errors on their surfaces. These errors come about because of the small tools removal characteristic and the misfit between rigid tools and aspheric surface. A passive semi-rigid tool, which can smooth ripple errors efficiently, is introduced in this paper. The tool is composed of a rigid layer, a compliant layer and a polishing layer. The passive tool works like a high-pass filter, smoothing the ripple out without damaging the surface figure. The expression of polishing pressure under the lap is deduced based on elastic plate theory. The relationship between the basic parameters of the lap and error frequency is also discussed. To testify the smoothing ability of the