Abstract: In order to reveal the impact mechanism of groundwater decline on tunnel water inflow under drainage conditions and improve the prediction accuracy, a calculation model for unstable seepage in tunnels considering drainage conditions is constructed. The water inflow is calculated based on Dupuit assumption and hydraulic theory, and the rationality of the model construction and the correctness of the calculation formula for water inflow are verified based on degradation analysis and engineering cases. Finally, sensitivity research is conducted on characteristic parameters, and the impact of drainage on tunnel water inflow is revealed. Research shows that when the groundwater decline is considered, the calculation error of tunnel water inflow can be reduced from 10.7% to 3.6%, from 13.3% to 3.1%, from 7.6% to 4.4%, and from 11.6% to 4.2%, which to some extent improves the prediction accuracy of tunnel water inflow; The height of the water head decreases nonlinearly over time. During tunnel water inflow, the drainage of the tunnel continuously affects the groundwater level, and the change of groundwater level should be considered in the calculation of tunnel water inflow; During the construction period, the water level continuously decreases, and the water inflow rate gradually decreases. The calculated total water inflow is larger than the actual value, and the design of the drainage equipment is relatively conservative.