When generate the four-axis NC machining code for the complex expanded contour of the outer cylindrical surface, there are problems of instability, time-consuming and low efficiency.Based on the shaft part which outer cylindrical surface has complex expanded contour, the method of converting the three-axis code of the expanded contour to the four-axis code was studied. Based on the structure and motion control mechanism of the four-axis CNC machine tool, the operation method of NC code generation for the outer cylindrical surface with complex expanded contour in UG software was analyzed.The data conversion relationship between the tool position coordinates of the three-axis NC code from the complex expanded contour and the four-axis NC code was researched, and a four-axis code conversion software for the shaft part with the cylindrical surface was developed. Through the specific operation of UG and the developed code conversion software, the four-axis NC machining code of the shaft part with the outer cylindrical surface of the complex expanded contour could be generated and simulated in Vericut software. The test result shows that this proposed method can be used to directly realize the four-axis NC code generation for the shaft part with the outer cylindrical surface of the complex expanded contour, and the stability and speed efficiency are verified.