The characteristic of servo system is an important data to influence the mechanical working properties of NC machine tool. In this paper, based on servo system characteristic, the cause of following error has been studied. It discussed the effect on machining precision of following error on part contour when machining circular contour. Then some calculation equations have been worked out. In order to reduce the following error, the method of compound control has been advanced in the last, and it has been proved to be a feasible way to reduce the influence of follow error on machining contour of NC machine tool.