Abstract: AISI 4340 bolt generated fractured at rest. After appearance inspection, macroscopic observation, microscopic observation, energy spectrum analysis, metallographic observation and hardness detection, the fracture properties of the fixed bolt were determined, and the causes of the fracture failure were analyzed. The results show that the fracture behaviour is resulted from hydrogen embrittlement. The influence of the bolt installation process on the hydrogen embrittlement has been analyzed in details. It is proposed that the proper bolt material chosen with given installation and heat treatment techniques can greatly reduce the risk of bolt fracture due to hydrogen embrittlement.