The removal function in ion beam figuring (IBF) exerts great influence on the fabrication residual error of optical surface, so that the performance of optical surface is changed because of the surface scattering induced by the fabrication residual error. In order to reduce the influence of the fabrication residual error on the optical performance, the IBF simulation procedure based on the computer controlled optical surfacing (CCOS) technique was used in surfacing the optical mirrors, and the influence of the removal function width on the ratio of scattering loss (RSL) was analyzed. It was shown that the influence of the fabrication residual error on the RSL declines with decrease of the removal function width. Thus the removal function width can be chosen reasonably according to the influence of the fabrication residual error on the RSL and the requirement of RSL index, and the processing technique can be optimized under the condition of satisfying the surface accuracy requirement of the optical mirrors.