Abstract: A safety valve on top of V401 coking unit of a chemical plant did not return to its seat after the start-up. After disassembly of the safety valve, it was found that its spring was broken. The fracture cause of the safety valve spring was analyzed by methods of chemical composition analysis, hardness testing, microstructure and fracture morphology observation, and corrosion product analysis. The results show that the stress corrosion of the safety valve spring occurred in a wet H₂S environment and then cracks were formed. When the effective cross-sectional area of the spring was not sufficient to withstand the fluctuation of the internal pressure of the medium, the spring failed, which belonged to corrosion fatigue fracture.