Abstract: A 20Cr2Ni4A steel shaft was carburized twice and cracked after standing for four d (days). The cracking causes of the steel shaft were analyzed by means of macro analysis, chemical composition analysis, fracture micro analysis, metallographic examination, blowhole and inclusion analysis. The results show that the unreasonable heat treatment process caused the excessive internal stress of the steel shaft, and the stress concentration was formed at the defects such as inclusion and porosity. The cracks initiated and propagated at the defects, which eventually led to the static cracking of the steel shaft.