Abstract: H13 steel casing was cracked after hot charging. The cracking reason was analyzed by means of chemical composition analysis, mechanical properties testing, fracture analysis, metallographic examination, hardness testing and so on. The results indicated that overheating phenomenon and heat treatment insufficient occurred in the forging process of raw materials, and it induced the excessive hardness of H13 steel casing, and toughness greatly reduced. Also, there was a greater residual stress in the tissue. At the same time, the outward expansion stress about steel casing in the assembly process was larger than material cracking resistance, so the casing was cracked in the early period.