Abstract: By using mechanical property testing and microstructure observation, the effect of quenching medium on the microstructure and mechanical properties of 40CrNiMo steel mining sprocket shaft after quenching and tempering was studied. It was found that the metallographic structure of the tempered sample after oil quenching was dense and uniform tempered martensite, with high strength, hardness, and good plasticity and toughness. The research results can provide theoretical basis for the design of heat treatment process for mining sprocket shafts and have reference value for guiding actual production.