Abstract: The cracking mechanism during quenching of 42CrMo steel plate was analyzed by microstructure analysis and mechanical testing, and the quenching process was optimized. The results show that the quenching cracking is due to the excessive cooling rate during quenching, the steel plate produces high internal stress, resulting in brittle martensite microstructure cracking. The parameters such as cooling speed, red return temperature and water flow ratio were optimized,the quenching cracking problem was solved, and good plate shape and mechanical properties were obtained at the same time.