A mathematical model of the computer numerical control (CNC) heavy cutting servo system including chatter in cutting courseswas constructed for the chatter in CNC heavy cutting. The theoretical analysis, computer simulation and orthogonal tests on thismodel show that increasing the gain of position Kpp can improve the rapid tracking performance of machine tools, and decreasing the delay time of speed loop τs can quickly eliminate the static error in the system, but the limited cutting width blim will descend correspondingly; excessively large or excessively small gain of speed loop Kps can result in decreasing blim; optimizing Kpp, τs and Kps can improve the dynamic and static performance of the system and increase blim. It is easy and feasible to op timize the servo parameters by the orthogonal test. This method can effectively improve the systemps stability and limited cutting width and it is suitable for the CNC heavy cutting of heavy-duty machine tools.