Abstract: In view of the locking spring fracture of the clamp braking unit in an EMU after running for 430000 km, detailed research and analysis from the aspects of stress state, fracture characteristics, production and assembly process, test verification, and so on is carried out in this paper. The results show that the fracture of locking spring is affected by external force during service, resulting in surface extrusion damage, forming crack source, and then gradually expanding until fatigue fracture. According to the analysis results, the structure and material properties of the existing locking spring are optimized, and the fatigue performance and reliability of the product optimization scheme are significantly improved through simulation analysis and test.