Abstract: An inlet valve of hydrogen is sealed with a copper valve gasket, which cracked after a period of operation. Through the analysis of the structure and working principle of the intake valve, the stress state of the valve gasket in the process of working was clarified. By analyzing the appearance characteristics and surface morphology of the failed valve gasket and comparing the macro morphology, metallographic structure and microhardness of the valve gasket before and after failure, the failure cause and mechanism are found as follows. Unreasonable assembly of valve gasket is the main cause of failure. The exist-ence of assembly clearance causes cyclic extrusion and deformation between valve seat and cylinder. At the same time, the oxide scale produced by gas corrosion is pressed into the surface of valve pad under the extrusion of cyclic force, forming “crack” defects. Deformation increases hardness and decreases toughness. The valve pad is broken at the weak point of the “crack” defect under the action of a larger impact force.