Abstract: For miniature circular disc specimens fabricated from isotropic, homogeneous metallic materials following the bilinear hardening rule, compressive loads are applied normal to the central circular surface of each specimen via a cylindrical planar small punch indenter. Based on the energy density equivalence principle and dimensional analysis, an elastoplastic semi-analytical model is derived to calculate the equivalent stress and equivalent strain at the midpoint of the Representative Volume Element (RVE). Distinct from conventional numerical inversion frameworks, the proposed model enables direct acquisition of equivalent stress–strain relations without prior knowledge of material constitutive parameters or iterative computations.Finite element simulations are carried out on sixteen pre-defined material constitutive datasets featuring varying elastic tangent moduli (E_T) and yield strengths (σ_y). To further validate the efficacy of the proposed method, Cylindrical Planar Small Punch Testing(CP-SPT) and standard uniaxial tensile tests are performed on three typical bilinear hardening metallic alloys, namely 316LN stainless steel, T225 alloy, and 40CrNi2MoV high-strength steel.Experimental results demonstrate that the stress–strain curves predicted by the CP-SPT technique achieve excellent consistency with uniaxial tensile measurements across the entire plastic deformation regime. The maximum prediction errors for elastic modulus and yield strength are controlled below 7.5%, which verifies the reliability and favorable predictive accuracy of the presented approach under practical test conditions. Furthermore, comprehensive discussions and future prospects are elaborated from eight critical perspectives: model parametric sensitivity, constitutive inversion performance, tolerance to random measurement noise, boundary condition influences, benchmark comparisons with existing Small Punch Testing (SPT) methodologies, material compatibility range, structural service loading conditions, and computational efficiency costs.This study establishes a novel small punch testing method tailored for bilinear hardening metals, which boasts a rigorous theoretical foundation, eliminates iterative inversion procedures, and directly extracts full stress–strain constitutive relations from raw experimental load-displacement curves. The developed technique delivers prominent practical engineering value for mechanical property characterization of miniature components and in-service structural materials under limited sampling conditions.