Abstract: The safety valve spring of a heavy oil catalysis cracking unit in a Guangzhou petrochemical plant fractured in several places, which caused the entire unit to be stopped unplanned. The fracture causes of the spring were analyzed by macroscopic inspection, chemical composition analysis, hardness test, metallographic examination, and fracture scanning electron microscope and energy spectrum analysis. The results show that the spring surface was exposed to the wet H₂S environment for a long time, the microstructure of the spring fracture zone was hard and brittle martensite, and the spring was subjected to high shear stress during the working process. Under the joint action of above three factors, stress corrosion cracks originated on the spring surface at the segregation and inclusion area. The cracks continued to expand and eventually led to fracture failure of the spring.