Abstract: In order to improve the forming quality of the spline and reduce the cost of die test, the CAE model of the spline part of the small gear was established, and the influence of the sizing belt length on the offset and underfill defects was analyzed. The spline was formed by the closed cold extrusion method, and the radial deviation trend of the spline was judged according to the contour map of the velocity field, and the full filling of the spline tooth shape was determined by the visualization of the simulation results and the minimum distance. The results show that the quality of the formed spline is the best when the length of the sizing belt is 3 mm. The results of this paper have a certain guiding significance for the actual production.