Abstract: To clarify the groundwater seepage characteristics of reservoir-adjacent tunnels in operation stage and evaluate the influence of grouting-ring design parameters on seepage control, the outlet section of the Zhujiashan Tunnel adjacent to a reservoir was selected as the engineering background. The target mix proportion of the similar material was determined through mix proportion tests and permeability coefficient tests. Indoor seepage model tests and full-scale numerical simulations were then jointly conducted to investigate the groundwater seepage characteristics around the tunnels in operation stage under different burial depths and reservoir water levels. The effects of the permeability coefficient and thickness of the grouting ring on groundwater seepage and tunnel seepage-control performance were further examined. The results show that: (1) The seepage intensity of reservoir-adjacent tunnels is jointly governed by burial depth and water level. When the relative height difference between the groundwater level and the ground surface remains unchanged, the drainage discharge exhibits a non-monotonic trend of first increasing and then decreasing with increasing burial depth. When the burial depth remains constant, the drainage discharge increases linearly with rising water level. (2) The water-pressure contours in the surrounding rock near the tunnel present a funnel-shaped distribution, and the downward deflection of the water-pressure contours above the tunnel centerline is more pronounced than that below it. Increasing either the permeability coefficient ratio of the grouting ring to the surrounding rock or the thickness of the grouting ring can effectively weaken this funnel-shaped distribution, reduce the drainage discharge, and improve the compression state of the secondary lining. (3) The numerical simulation results agree well with the model test results in terms of both spatial distribution and variation trends. Based on the numerical simulations, the optimized parameter combination suitable for the background project is a grouting-ring permeability coefficient ratio of 50 and a grouting-ring thickness of 6 m.