Abstract: Fracture failure occurred during the assembly process of a high strength bolt. The reasons for the fracture failure of the bolt were analyzed by macroscopic observation, chemical composition analysis, mechanical property test, metallographic examination and scanning electron microscope analysis. The results show that the hardness of the bolt was high, and the radius of the transition round angle of the joint of head and rod was small. In the process of the assembly of large torque, the combined stress of torsional and bending causes fracture failure of the bolt.