<p>To accurately predict tunnel water inflow under complex geological conditions, this study established a unified geomechanical calculation model for such environments. Using groundwater dynamics and Bernoulli's energy equation, a theoretical approach to calculate the tunnel face water inrush under non-Darcy flow conditions was proposed, which comprised a confined water formula and a phreatic water formula. This methodology considers temporal and geological variations to enable short-, medium-, and long-term dynamic predictions of water inflow. The research findings indicate that: for short-term predictions, the theoretical water inflow derived from both formulas exhibited at most 5% deviations from the actual measurements. When a 90% confidence interval was adopted for the medium-to-long-term predictions, the relative errors for both maximum and normal water inflow remained below 5%. Notably, during the decaying water inrush phase, the phreatic water formula predicted a 2–3 times slower decay rate than the confined water formula. The maximum water inflow positively correlates with the permeability coefficient (<i>K</i>), water head height (<i>H</i>), and diameter of the water inrush channel (<i>d</i>), whereas it negatively correlates with the length of the water inrush channel (<i>L</i>). Conversely, the normal water inflow negatively correlates with <i>K</i> and <i>d</i> but positively correlates with <i>H</i> and <i>L</i>. After the feasibility of this method had been validated in the Yongshun Tunnel and Zhongliangshan Tunnel, it was successfully applied to a mountainous tunnel project in southwestern China.</p>