Abstract: A CrWMn steel piercing mould cracked around the round hole near the edge after it was polished on a grinding machine. Besides the analysis of heat treatment process of the cracked mould,the cracking reason was also analyzed by using the high and low multiple metallographic structure observation and chemical compositions analysis on the heat treatment test sample and the raw material. Verification test was performed to prove the reason. The results show that the crack resulted from the improper heat treatment processing. The mould was not precooled before quenching. Then it was cleaned with cool water when it didn’t cool completely after quenching. In addition,the mould was not tempered fully. All these improper processing resulted in a larger internal stress and structure stress which made the mould crack.