<p>Seepage through fractures at the granite–rock interface has a significant impact on the performance of hydraulic structures. In this study, ten sets of granite–concrete interface fracture samples were divided into two groups and a series of seepage experiments was conducted under different seepage pressures and erosion solution concentrations to investigate the influence of multi-factor coupling on the seepage mechanism. The effects of seepage pressure and erosion solution concentration on seepage flow, permeability, and calcium ion dissolution at the granite–concrete interface were quantitatively evaluated. Additionally, this study explored the evolutionary mechanism of fracture permeability at the interface. The results demonstrate that, across all tested conditions, the seepage flow and fracture permeability initially decrease rapidly, followed by a slower rate of decline before eventually stabilizing. It was observed that higher seepage pressures accelerate erosion under identical conditions. As the concentration of erosion solution increases, the maximum reduction in permeability reaches 91%. A linear relationship was established between permeability and erosion solution concentration, as well as between seepage flow and the pressure gradient, with R² values of 0.9739 and 0.9211, respectively. The cumulative dissolution of calcium ions was found to initially rise rapidly before stabilizing over time. The peak dissolution of calcium ions occurred at an erosion solution concentration of 7%, reaching 12.07&#xa0;g/m³. Under seepage pressure, the peak dissolution rate of calcium ions was 0.088%. These findings elucidate the coupling mechanism and quantitative patterns of seepage erosion at interfaces under complex conditions, providing a theoretical basis for the scientific estimation of fracture permeability in granite–concrete systems.</p>