After carburizing heat treatment, a modified layer forms on the surface of alloy steel, within which the mechanical properties of the material change along the depth direction. In this paper, a delamination test method for measuring Johnson–Cook damage parameters is proposed for the surface modification layer of 18CrNiMo7–6 alloy steel after carburizing heat treatment. Thin plate samples with different metamorphic layer depths were obtained by wireelectrode cutting from the modified layer of the material. In order to obtain tensile results under different stress triaxiality, this work prefabricated tensile shear specimens with different fracture directions, and determined the stress triaxiality considering the cumulative effect of strain through combination of experimental and simulation methods. In addition, the relationship between material failure strain and stress triaxiality was measured and the relationship between failure strain and strain rate of tensile specimens was measured using a wide pulse tensile system. The results show that the failure strain of a material in the same layer decreases with the increase of stress triaxiality, and at the same stress triaxiality level, the failure strain increases with the increase of depth. The samples at different layer depths had significant strain rate weakening effects, and at the same strain rate level, the failure strain increased with the increase of layer depth. Based on the measured parameters, a numerical simulation of the tensile process was conducted and compared with the experiment to verify the accuracy of the parameters measured in this paper.