Computer controlled optical surface (CCOS) technology is a very important method to fabricate large aspheric optical surfaces. Computer numerical controlled small tool technology is based on Princeton hypothesis. It used a small lap which travels over the work-piece surface. By accurately controlling the dwell time and other parameters,a prescribed amount of material can be removed at each point on the surface. There is the problem in the process of manufacturing. Aspheric optical elements,which fabricated using computer controlled optical surface technology,almost have medium high-frequency(≤33 mm period) errors on their surfaces,and very difficult to remove . A flexible tool is fabricated in order to deal with the problem. The transmutation of the flexible tool has been simulated. The thickness of the two kind of flexible material has been calculated by the simulation. The flexible tool had been used in the manufacture process of 420 mm f/2,paraboloidal mirror. After three hours of smoothing with the flexible tool,almost all medium high-frequency errors have been removed,and the precision of aspheric optical surface isn′t destroyed.