Abstract: The return spring of pneumatic control valve in multiple unit train broken abnormally several times, affecting the safety of the train. In this paper, the failure of the spring was analyzed by means of macro and micro morphology observation, SEM and EDS analysis, metallographic microstructure analysis and Vickers hardness testing. The results show that the fracture of the spring is low cycle fatigue fracture, and the cracks initiated from the transverse scars or pits on the inner ring surface. During the service, the cracks propagated from the inner ring surface to the outside, finally causing the early fracture failure of the return spring. In this paper, it is considered that the transverse scars and pits on the surface of the spring inner ring were formed in the manufacturing process, and this was further confirmed by the retrospective verification of manufacturing tooling. It is suggested that the guiding fixtures should be made of softer materials with smaller friction coefficient, and the fixtures used in the manufacturing process should be regularly overhauled.