The dual effects of seepage control and load bearing of the grouting curtains in pregrouting reinforcements of deep tunnels are difficult to quantify under high water-pressure conditions. To address this theoretical challenge, a fluid-solid coupled elastoplastic model incorporating grouting curtains under non-Darcy flow conditions is constructed. The analytical solutions for the ground reaction curve (GRC) and plastic-zone radius are then derived and validated through finite-element simulations. Finally, the influence of the grouting parameters on GRC evolution is systematically investigated. The results demonstrate the following: (1) Increasing grouting curtain thickness and reducing its permeability coefficient notably decrease tunnel water inflow, while non-Darcy effects substantially compromise seepage control efficiency. (2) Compared with Kastner’s solution, pregrouting dramatically reduces the radial displacement of surrounding rock at the elastoplastic interface under seepage and non-seepage conditions. (3) The anti-seepage efficiency decreases with increasing curtain thickness, suggesting an economic threshold of ~1.5 times the tunnel radius to avoid overdesign. (4) Increasing the shear modulus reduces elastoplastic boundary displacement and increases rock stiffness in the GRC elastic segment. Increasing the cohesion and internal friction angle dramatically decreases the required support pressure to maintain an elastic state. (5) A larger excavation radius intensifies deformation and support demand, while the shear angle primarily governs plastic deformation.