Abstract: The local geotectonic setting is a key factor influencing fault activity. To address the unclear mechanism of injection-induced fault reactivation under the combined effects of critical stress states and internal permeability structure, this study investigates industrial wastewater reinjection using a 3D hydro-mechanical coupled conceptual model of a dominant fault, developed through numerical simulation. Based on the Mohr-Coulomb criterion and slip-weakening law, the timing and displacement of fault reactivation were calculated, and the influence of critical stress states and internal permeability structure on the hydro-mechanical response of the fault was analyzed. The results indicate that pore pressure diffusion from fluid injection alters the hydro-mechanical behavior of surrounding strata, but this behavior is controlled by the initial in-situ stress state and the fault's permeability structure, with the latter being the dominant factor. As the fault evolves, the proportion of low-permeability fault core material increases, leading to a more pronounced hydro-mechanical coupling response under any critical stress state and enhanced pore pressure accumulation. This study recommends that industrial wastewater reinjection be prioritized in low-permeability fault zones within strike-slip stress regimes, using a long-term, low-rate injection strategy to reduce the risk of fault reactivation.