Hardness is an index that reflects the comprehensive mechanical properties of materials, but it is not sufficient for characterizing the specified mechanical properties (e.g., elastic modulus, yield strength, strain hardening exponent, etc.) or constructing the material constitutive model. Especially for the carburized gears, due to the gradient distribution of the mechanical properties, it is difficult to obtain the constitutive relationship parameters of the surface hardened layer by the existing methods, e.g., tensile mechanics test. In this paper, the surface hardened layer was cut and polished layer by layer, and the constitutive relationship parameters such as elastic modulus, yield strength, strain hardening exponent and tensile strength of each layer were tested and characterized by the flat indentation method, and the Vickers hardness was predicted by incorporating the energy equivalence theory. By analyzing the evolution laws of the constitutive relationship parameters and the Vickers hardness, a simple formula describing the relationship between them was established along with the hardened layer depth. The results show that the elastic modulus almost does not change with the hardened layer depth, and the strain hardening exponent, tensile strength, and Vickers hardness all gradually decrease with the hardened layer depth. The method of this paper provides a simple and effective new method for testing and characterizing the mechanical properties of the hardened layer of carburized gears, which is of great significance for the quality assessment and strength analysis of carburized gears.