Accurately characterizing the damage part surrounding a roadway under anisotropic in-situ stress remains a critical challenge in deep underground engineering. In this study, an approximate implicit formulation of the plastic zone boundary is developed by integrating elastic stress fields—derived from classical elasticity theory—into a plastic yield condition. Initially, the scenario with a lateral pressure coefficient of unity is employed as a reference to validate the approximation. A comparison with the analytical solution based on the axisymmetric plane strain model reveals significant deviations in the estimated plastic zone radius, particularly in geomechanical environments involving deep and weak surrounding rocks, where the approximation may become unreliable. To further evaluate the model’s applicability, finite element simulations are utilized to depict the actual distribution of the plastic zone. The morphological features obtained from the implicit formulation show strong agreement with the simulation results, despite notable discrepancies in absolute size. These findings suggest that, although the proposed analytical method lacks precision in quantifying the extent of plastic deformation, it remains effective for capturing the diameter of the damage zone. This provides a practical tool for assessing deformation characteristics in rock masses under unequal biaxial compression.