Abstract: This study presents a semi-analytical model for the efficient characterization of the stress field at the tip of a three-dimensional Mode I crack. The research addresses key limitations in existing elastoplastic fracture models—such as the HRR solution and J-Q theory—which typically rely on computationally intensive finite element analyses and lack explicit analytical formulations. By applying the principle of energy density equivalence, an analytical expression for the equivalent stress at the tip of a planar Mode I crack is first derived. Subsequently, systematic three-dimensional elastoplastic finite element simulations demonstrate that, when normalized by the equivalent stress at the median point of an energy density equivalent unit, the crack-tip stress distribution is predominantly governed by two dimensionless parameters: the relative crack length (a/W) and the specimen thickness ratio (B/W). Based on these, a Crack-tip Equivalent Stress Field for 3D Mode I Crack (CESF-3DMIC) model is developed. The model adopts a compact power-law formulation, with its key parameters explicitly defined as functions of a/W and B/W. Specific parameters are provided for both compact tension (CT) and single-edged bending (SEB) specimens. Validation against three-dimensional finite element results confirms that the CESF-3DMIC model accurately predicts crack-tip stress distributions in both specimen types. In contrast to conventional methods such as the J-Q approach, which require finite element-based regression techniques, the proposed model offers a direct, parameter-explicit analytical framework. It enables rapid prediction of the crack-tip stress field without reliance on complex numerical computations, thus serving as a practical tool for fracture safety assessments and theoretical developments incorporating geometric constraint effects.