Abstract: The sample of 42CrMoA round steel produced by a certain steel plant showed cracking phenomenon after quenching and tempering treatment. The causes of cracking in the sample were analyzed using methods such as macroscopic observation, chemical composition analysis, metallographic examination, hardness testing, scanning electron microscopy and energy spectrum analysis. The results show that the surface cracks of the sample were quenching cracks, and the crack source was located on the side near the center of the base material. The combined effects of short quenching insulation time, uneven organization of raw materials, segregation of Cr element composition and sulfide aggregation in the segregation zone led to quenching cracking of the sample.