Abstract: The fracture causes of a transmission spline shaft sleeve were analyzed by means of optical microscope, scanning electron microscope, microhardness tester.The results show that the chemical composition and hardness of the spline shaft sleeve are in accordance with the requirement.The stress corrosion cracking and propagation occur in the spline shaft sleeve due to the long-term action of residual corrosive medium and stress.Under the coupling action of superimposed welding stress and suit stress, the penetrating crack forms, which results in the decrease of load performance of the spline shaft sleeve, and finally torsion fracture occurs at the penetrating stress corrosion crack tip under the action of torque.