Abstract: The high pressure main steam valve stem of a turbine broke during the operation of the unit. The fracture reasons of the stem were analyzed by means of macro observation, chemical composition analysis, metallographic examination, scanning electron microscope analysis and mechanical property test. The results show that the improper heat treatment process of the stem material results in the abnormal structure, unqualified strength and low impact toughness of the material. There was an interference fit between the stem and the sleeve. In the case of oxide skin formed on the surface, the interference degree was further increased. Under the action of large tensile stress, stress concentration was generated, and overload cracking occurred at the cross hole of the stem, which eventually led to the fracture of the stem.