Abstract: The fracture causes of 42CrMo steel axle box end cover were analyzed by means of macroscopic analysis, chemical composition analysis, mechanical properties test, metallographic examination and SEM fracture analysis. The results show that the fracture of the end cover of the axle box was a multi-source vibration fatigue fracture. The main reason for fatigue fracture was that under the stress of alternating tension and compression vibration, the stress concentration occured at the circumference of the inner corner of the bolt sinkhole, resulting in multi-source vibration fatigue cracks.