On the basis of Preston hypothesis,the motion relationship between tool and workpiece upon the tools motion in planar model is analyzed.The effect on computer controlled optical surfacing (CCOS) caused by controllable variable is simulated except for the dwelling time,thus,some reference on theory is provided to optimize the former numerical control (NC) model,and fast manufacturing of large departure aspherics is realized. On the basis of Preston hypothesis, the motion relationship between tool and workpiece upon the tool s motion in planar model analyzed. Effect on computer controlled optical surfacing (CCOS) caused by controllable variable is simulated except for the dwelling time, thus, some reference on theory is provided to optimize the former numerical control (NC) model, and fast manufacturing of large departure aspherics is realized.