Abstract: The fixing bolt of the undercarriage brake disc of an aircraft was found to be broken during the maintenance inspection before the aircraft took off. The fracture causes of the bolt were analyzed by macro analysis, micro analysis, metallographic examination, hardness test and hydrogen content determination. The results show that the fracture of the fixing bolt had the characteristics of hydrogen brittleness fracture, and the fracture failure property was hydrogen brittleness fracture. Environmental corrosion of the local bolt occurred in service. The hydrogen produced by corrosion was absorbed by the bolt, which led to hydrogen brittleness fracture of the bolt.