<p>This study investigates the effect of excavation-induced phreatic line drawdown on seepage discharge in mountain tunnels situated in horizontal strata. By integrating the Dupuit hypotheses with the Boussinesq equation, theoretical solutions were developed to describe the equilibrium state phreatic line and quantify the corresponding seepage discharge. These solutions were derived using the integral method and were further analyzed to identify potential sources of error. To validate the theoretical framework, numerical simulations were conducted using ABAQUS, revealing strong correlations and offering insights into discrepancies. Key parameters influencing the equilibrium state phreatic line and seepage discharge were systematically examined, including hydraulic conductivity, the horizontal distance from the tunnel’s central axis to the lateral boundary of groundwater recharge, recharge height, the vertical distance from the tunnel’s central axis to an impervious layer, tunnel radius, and the thickness of horizontal strata. The applicability conditions of the theoretical solutions were also established through comparative analysis with numerical results. Despite the notable relative errors inherent in the theoretical solutions, they account for phreatic line drawdown caused by tunnel excavation, offering an analytical tool for preliminary design and seepage evaluation in tunnel engineering.</p>