Abstract: A traction motor cast aluminum structural part cracked and failed during the long-life vibration test.The failure mode of the cast aluminum structural part was analyzed by the fracture morphology analysis, chemical composition detection, microstructure observation, hardness detection, and strength simulation analysis.The results show that the failure mode of the cast aluminum structure is fatigue fracture, and the crack source originates from the bottom of the groove of the end cap.The reason of fracture is that the structural part in this place is relatively thin, only 4mm.In addition, there are casting defects, the material continuity is interrupted, and the effective bearing strength is weakened. And this place is a right-angle transition, forming stress concentration under the use of stress.By increasing the thickness of the groove, the right-angle transition of the groove is changed to a rounded transition, which reduces stress concentration and improves the safety factor of the cast aluminum structural part.