<p>An improved inverse analysis model for the single self-similar indenter was proposed to extract the stress–strain response. A duplex lightweight steel was chosen as the model material; however, the improved model can be widely applied to inverse analysis of different materials. The stress–strain response was described by the well-known Hollomon equation, where the yield strength and the strain hardening exponent were obtained via the improved model, and the elastic modulus was calculated through the classical O&amp;P method. The improved model was established based on the typical strain energy equivalent principle and cavity theory. The effective deformation volume and equivalent strain in above framework are identified with the residual indentation profile and the complete load–depth curve of the indentation, rather than relying solely on the loading stage curve. This improvement replaces empirical fitting coefficients with clearer expressions and avoids the cost of complex finite element iterations. The macroscopic stress–strain curves of the samples were fitted by inverse analysis results, and the calculated results fell within a credible fitness range, which verified the accuracy of the model prediction. The robustness analysis also shows that the model can ensure stable calculation accuracy within a wide range of constitutive parameters.</p>