Lightweight automobiles have a lower impact on the environment and save energy; therefore, they have become a focus within the automobile industry. Hot stamping parts made of high-strength steel have been widely used in car bodies. To study the mechanical properties and constitutive model of high-strength steel after hot stamping, the samples containing full martensite, full bainite, and martensite/bainite dual phases structure were obtained by controlling the tool's temperature and holding time during hot stamping. Then the load-displacement curves of different microstructures were obtained using nanoindentation tests. Subsequently, the modulus of elasticity, yield stress, strain hardening exponent, and other mechanical properties of these microstructures were calculated by reverse algorithms using dimensional analysis. Further, the power-law elastoplastic constitutive models of different microstructures were derived using these parameters. The errors of yield strength obtained using the reverse algorithm and tensile tests in full martensite and full bainite samples are -1.15% and 3.38%, respectively. The yield strength of the martensitic/bainite sample obtained using the reverse algorithm is 16.62%, 24.17%, and -11.78% different from that obtained by the tensile test, showing that the mechanical properties are different under macroscopic and microscopic conditions to some extent. Simultaneously, the average yield strength of the three points is only -1.41% different from that obtained using the tensile test. Finally, the derived constitutive models were verified by simulating the finite element nanoindentation. The results show that the constitutive model obtained using the inverse algorithm can accurately describe the mechanical properties of the main microstructures of high-strength steel after hot stamping.