Computer controlled optical surfacing (CCOS) technology is a common method to fabricate off-axis aspheric mirrors,whose diameters are less than 500 mm. CCOS technology is based on Preston hypothesis. It uses a small lap which travels over the workpiece surface. By accurately controlling the dwell time and other parameters,a prescribed amount of material can be removed at each point on the surface. Considering the problem in the process of manufacturing,a best fit sphere is chosen reasonably. A computer controlled model is found,which tool path is along the contour line of sagittal surface of the off-axis aspheric mirror. The two dimensional convolution operation is reduced to one dimensional convolution. The constant-linear pressure distribution model is used to analyze the edge effect. The manufacture process of a 124 mm off-axis aspheric mirror has been simulated. The result shows that under the direction of the model,the root mean square of its surface figure error decreases from 0.018mm to λ/60 (λ=632.8 nm).