Abstract: When installing and fastening a hexagon head bolt, the fracture occurred at the lower fillet of the bolt head. The fracture causes of the hexagon head bolt were analyzed by means of macroscopic observation, chemical composition analysis, macroscopic examination, metallographic examination, mechanical property test, scanning electron microscope and energy spectrum analysis. The results show that there were defects in the head of the hexagon head bolt before leaving the factory, which led to stress concentration and quenching cracks during quenching. In the case of existing cracks, the bolt was subjected to external stress during installation, which caused the quenching crack to expand, resulting in the final fracture of the hexagon head bolt.