Abstract: Obtaining the microstructure of different pre-treatment surfaces of 42CrMo nitrided steel through normalizing and quenching and tempering treatment, OM, EDS and microhardness tester were used to analyze and detect the samples after ion nitriding, and a portable microhardness tester was used to verify the actual production of nitrided steel. The results indicate that when there is a semi decarburized layer structure on the surface of 42CrMo nitrided steel, the microhardness test results of the sample with normal non decarburized structure on the surface are similar, and the combination of nitrogen and alloying elements in steel to form alloy nitrides dominates the strengthening effect, However, its surface brittleness level has significantly decreased, and the decrease in hardness gradient at equidistant distances from the surface is the largest. The microstructure level and matrix hardness are the main factors affecting the surface Vickers hardness after ion nitriding detection.