Abstract: MasterCAM 9.1 software provides a variety of programming methods for four-axis machining. Among them, the replacement axis programming method has been widely used in practical processing. It has been proved to be an effective programming method in practical production. However, if this method is not applied properly, it will lead to over-cutting or errors in parts processing. Taking cylindrical groove programming as an example, this paper finds out the causes of parts overcutting or machining errors caused by MasterCAM 9.1 software replacement axle programming method, and analyses them. It effectively solves the problem of parts overcutting or machining errors caused by replacement axle programming method.