Abstract: In the field of material properties research, it is crucial to analyze the failure of metal parts under complex working conditions. For example, a hexagonal head bolt of M12×64 specification made of 1Cr17Ni2 steel fractured accidentally during the vibration test after completing the high-temperature and low-temperature impact tests. In order to investigate the fracture mechanism in depth, this study comprehensively uses a series of means, such as chemical composition analysis, macroscopic and microscopic fracture morphology observation, microstructure evaluation and mechanical property testing, to systematically analyze the characteristics and root causes of bolt fracture. The results show that the bolt has been subjected to a certain degree of bias load during actual use. Through detailed investigation, it was found that there were asymmetric signs of wear on the light bar part of the bolt, and the hexahedral surface of the head of the bolt had obvious wrenching traces, while the support surface of the head did not show any friction traces. This phenomenon indicated that the bolt failed to fit tightly with the contact surfaces after installation. Due to this non-ideal installation condition, the bolt was subjected to uneven forces during service, which significantly weakened the fatigue resistance of the threaded portion. Eventually, under the cyclic loading of the vibration test, the bolt suffered fatigue fracture.