Abstract: Early fatigue fracture occurred to disk springs of hydraulic spring operation system during service. The failure cause was analyzed by means of chemical analysis, metallographic examination, scanning electron microscopy and energy spectrum analysis. The results show that there existed a higher concentration of loose structure and pores at the junction of the concave inner surface and the end of the disc spring, and some of them broke through to the surface, forming a pre-crack. Stress concentration was caused by pre-cracks under the effect of maximum alternating shear stress. Pre-cracks acted as fatigue crack source. Fatigue crack propagated and resulted in the fracture. Defects such as folding, micro-cracks and scars at the surface of the disc spring also influence the security of disk springs.