In this paper,the input shaft of the agitator 34-M-2001 in a polypropylene device is studied.For the fracture problem of the input shaft in the same key groove,the strength of the shaft,force distribution of the key grooves and its factors such as the processing technology are analyzed synthetically to determine the causes of the failure and propose the optimized improvement program.In the case of the impossibility to change the shaft diameter,the performance of the shaft can be improved by changing the chemical composition of the material or choosing a better material.Simultaneously,the optimization for reducing the stress concentration at the tail of the key grooves is proposed by changing the processing technology.The studies indicate that the improved shaft has an excellent performance.The above researches have a great reference value for improving the mechanical performance,service lift of the transmission parts,especially for the shaft.