Abstract: During the operation of a hydrogen diaphragm compressor, the cylinder head fractured. The fracture reasons were analyzed by means of macroscopic observation, chemical composition analysis, metallographic examination, scanning electron microscope analysis and mechanical property test. The results show that the improper heat treatment process led to the formation of unqualified microstructure, which significantly reduced the strength and toughness of the material. Metallurgical or forging defects (such as porosity, holes, etc.) in the material destroyed the structural continuity of the steel. Under the action of hydrogen environment, cracks gradually formed inside the material, which eventually led to brittle fracture.