In order to improve the production efficiency of a certain automobile transmission spline shaft and ensure the production quality and control the production cost, the overall structure of spline shaft was analyzed, and a six-station cold precision forging process was designed. Then, the cold deformation behavior of spline shaft material was studied, and a more accurate constitutive model was established for the simulation test. Furthermore, the precision forging process of spline shaft was analyzed in detail by software Deform-3D, and the folding defect in the fifth station were found. Finally, the cause of the defect was determined according to the metal flow process, and the mold cavities of the third station and the fourth station were reasonably optimized. The test results show that the optimized process can effectively solve the folding defect, and the obtained spline shaft has a complete shape and good internal quality. In addition, the cold precision forging test of the optimized process is carried out, and the produced spline shaft product has a good appearance, continuous cross-sectional streamlines, orderly distribution, and no obvious defects, which is highly consistent with the simulation results and verifies the effectiveness of the optimized process.