Abstract: The valve spring in an automobile engine broke after running for 600 km. The fracture reasons of the valve spring were analyzed by means of macroscopic observation, chemical composition analysis, metallographic examination, hardness test, fracture analysis and CT detection. The results show that the fracture property of the valve spring was fatigue fracture. The shot peening process led to the pit defect in the spring, and the stress concentration in the pit was large. Under the condition of cyclic force, the surface of the spring formed cracks, which continue to expand, and finally caused the spring to fracture.