Abstract: The failure analysis was conducted on the cracked rear torsion beam of a certain vehicle model after fatigue durability test by using direct-reading spectrometer, hardness tester, scanning electron microscope and metallographic microscope. The chemical composition and hardness meet the material standards and technical requirements of the drawings. The results of fracture and metallographic inspection show that the failure mode of the rear torsion beam is stress corrosion cracking originating from the inner wall. The main reason is that the poor anticorrosion treatment of the inner wall of the rear torsion beam causes corrosion, resulting in stress corrosion cracking and expansion of the rear torsion beam with high-strength tissue sensitivity under the tensile stress of the fatigue strength test. The fatigue durability test was carried out with the rear torsion beam assembly without corrosion on the inner wall, and no cracking problem was found.