<p>During rock mass excavation and unloading processes, the coupled effects of sudden confining pressure reduction and seepage frequently induce rock damage deterioration and water/mud inrush disasters. The current research remains insufficient in elucidating the coupled evolution mechanisms of mechanical energy-seepage interactions in pre-damaged rocks. This study systematically investigated the mechanical responses, energy evolution, and permeability variation patterns of pre-damaged red sandstone by performing triaxial compression tests and constant stress difference confining pressure unloading tests. A damage constitutive model based on energy dissipation was subsequently developed. Experimental results demonstrated that rock failure exhibited typical brittle characteristics. Stress–strain curves were divided into five stages, including crack closure, elastic deformation, stable crack propagation, unstable crack propagation, and post-peak stress drop. Damage accumulation was significantly accelerated by confining pressure unloading. Energy allocation was shifted from elastic storage to plastic dissipation. Under a high initial stress level of 90% σ_c, energy release was intensified, and abrupt instability was promoted. A nonlinear positive correlation was observed between permeability and the damage variable. A permeability surge was triggered after a critical damage level was reached, because seepage channels became interconnected. A damage constitutive model incorporating confining pressure unloading–seepage coupling was established based on energy theory and staged mechanical responses. Verification confirmed that rock deformation and failure processes were accurately characterized. These findings provide theoretical foundations for evaluating unloading-induced damage and for early warning of seepage-induced disasters in rock masses.</p>