Abstract: Fracture failure of a hot extruded mandrel for bearing ring of 3Cr2W8V alloy steel has occurred, the causes of fracture failure were analyzed by means of fracture macroscopic analysis, chemical composition analysis, hardness test, metallographic examination and so on. The results show that the structure design of the mandrel was unreasonable and the surface quality of the mandrel was bad, and the stress concentration was caused by the combined action of tension-compression alternating stress and torsional stress. The fatigue crack source was formed when the inclusions in the transition fillet parts were concentrated along the stress concentration, and early fatigue fracture occurred.