Abstract: The tube bundle of a circulating oil steam generator fractured. The failure causes of the tube bundle were analyzed by on-site inspection, return inspection and sampling inspection (chemical composition, microstructure, crack morphology, mechanical properties, fracture morphology and surface scale composition). The results show that the fracture failure of the tube bundle was caused by stress corrosion because of evaporation and concentration of the medium in the gap of the tube hole. Finally, according to the reasons for the failure of tube bundle cracking, some suggestions were proposed on design, material selection, manufacture and water quality assurance of the circulating oil steam generator tube bundle.