Abstract: To obtain the optimal heat treatment system for ZG40CrNi2Mo large gear steel，the microstructure, mechanical properties and impact toughness of the ZG40CrNi2Mo steel after heat treatment were studied by SEM, hardness tester and impact testing machine. The results show that the microstructure of the ZG40CrNi2Mo steel is lath martensite after quenching. With the quenching temperature increasing, the hardness increases first and then decreases, the grain size increases from 5 μm-15 μm to 15 μm-25 μm, and the austenite grain grows abnormally. When the quenching temperature is 840 ℃, the grains are fine and uniform, and the steel exhibits relatively high impact toughness of 65 J. After tempering at 560 ℃, the microstructure exhibits the characteristics of tempered martensite. With the tempering temperature increasing, the hardness decreases and the impact toughness improves. When the tempering temperature is 660 ℃, the impact energy is up to 76 J and the hardness is 281 HBW, which can meet the performance requirements of large gear rings. In summary, the optimum quenching and tempering temperatures for ZG40CrNi2Mo large gear rings are 840 ℃ and 660 ℃, respectively.