Abstract: The bolts on the low-pressure cylinder inner cylinder joint surface of a 330 MW supercritical steam turbine unit in a thermal power plant fractured during operation. The causes of bolt fracture were analyzed using macroscopic observation, chemical composition analysis, metallographic examination, mechanical property testing, scanning electron microscopy and energy spectrum analysis. The results show that there were cracks and a certain depth of decarburization layer at the top of the bolt thread, which reduced the strength of the thread. There were many non-metallic inclusions in the structure of bolt materials, which disrupted the continuity of the matrix. The plasticity of the bolt material was poor, caused microcracks to form at the root of the bolt. When the bolt was used in a corrosive environment, the cracks propagated under high stress, ultimately led to brittle fracture of the bolt at the first thread under high stress.