Abstract: The spring of pressure relief valve on a pressure vessel broke during service. The reasons for the spring breaking were analyzed by macro-inspection, fracture morphology analysis, metallographic analysis, mechanical property testing and chemical composition analysis. The results show that there are a large number of pitting pits on the whole spring surface, and the crack initiation area is located at the pitting pits on the inner surface of the spring. The residual tensile stress on the inner side of the spring itself is large, and the stress concentration caused by pitting pits accelerates the initiation of cracks. In addition, there are corrosive sensitive media such as S and Cl on the surface and cross section of the spring. The spring itself has high strength and hardness. Under the combined action of tensile stress and corrosive medium, stress corrosion cracking occurs at the stress concentration part caused by pitting pits, which eventually leads to the failure of the spring. It is suggested that the hardness of the spring should be properly reduced to reduce the sensitivity of stress corrosion cracking, and the surface coating process of the spring should be improved to improve the corrosion resistance.