Abstract: Macroscopic observation, chemical composition analysis, metallographic examination, mechanical property test and hardness test were used to study the fracture reason of main valve stem of a 300 MW steam turbine. The results show that there were problems with the door rod such as variable cross-section structure, relatively concentrated stress, inclusions in the interior, and low material hardness. Under the action of alternating stress, the door rod was subjected to great impact force, which exceeded its critical fracture strength, and finally brittle fracture occured.