Abstract: Aiming at the fracture failure of the crank shaft of the joint RV reducer after the robot has been running for a long time, this paper analyzes the causes of the fracture of the crank shaft from the aspects of material chemical composition, material properties, heat treatment hardness, infiltration layer structure and so on. The results show that the fracture surface of the crank shaft has typical fatigue fracture characteristics, and the stress concentration at the grinding return groove is the main reason for the fracture of the crank shaft. After optimizing and improving the grinding return groove through finite element simulation and completing the installation verification test, it shows that the optimization measures can effectively avoid the occurrence of crankshaft fracture and improve the reliability of RV reducer.