Abstract: The connecting bolt of draft tube inlet gate of a large hydropower station turbine fractured during operation. Means such as macro and micro morphology analysis, chemical composition analysis, metallographic examination and mechanical properties test were used to analyze the fracture causes of the bolt. The results show that due to improper heat treatment process of the bolt, serious decarburization layer was formed on the surface of the bolt, and the layer reduced the fatigue resistance of the bolt. There were more massive ferrites in the bolt matrix, which reduced the strength margin and toughness margin. Under the cyclic impact load caused by draft tube overflow, fatigue cracking was formed by the microcracks along the decarbonized layer, and then extended to fracture.