Water inrush disaster is the main geological disaster of water-rich and deep-buried karst tunnel, which is extremely harmful and threatens tunnel construction and construction safety. When building tunnels in water-rich and deep-buried karst areas, it has become a major challenge to evaluate the risk and study the mechanism of water inrush from tunnels. In order to solve this problem, this paper takes Nahecun water-rich deep-buried karst tunnel of Tian'e through Fengshan to Bama to Expressway as the research object, analyzes the hydrogeological characteristics of karst area through geological investigation, grasps the characteristics of groundwater occurrence and migration, finds out the main disaster-causing factors and influencing factors of water inrush disaster in karst tunnel, reasonably selects the influencing factors of water inrush and their weights to construct a combined weighting -TOPSIS method tunnel water inrush risk assessment system, verifies the evaluation system through field excavation experiments, and analyzes the water inrush mechanism of karst tunnel by using finite element analysis software. The research results show that the main influencing factors of water inrush in karst tunnel include stratum, geological structure, topography, hydrogeological conditions and other factors. The evaluation weight of each index is obtained by using analytic hierarchy process and entropy weight method, and the distance and relative proximity between the typical tunnel section and the ideal solution are obtained according to the combined weight -TOPSIS method, so as to evaluate the water inrush risk of Nahecun tunnel section. Based on the equivalent continuum model and Hoek-Brown yield criterion, a three-dimensional simulation model of Nahecun tunnel is established, and the pore water pressure of initial stress-seepage coupling and secondary stress-seepage coupling after excavation is carried out. It is concluded that the pore water pressure near the two sides of the cave is higher than that above, and the area with higher pore water pressure near the cave on the left is concentrated around the cave, while the pore water pressure on the right side is dispersed. The pore water pressure around the initial coupling cave is obviously higher than that around the secondary coupling cave, and the maximum pore water pressure on the left side of the cave is greater than that on the right side of the cave. The research results provide theoretical data support for water inrush risk assessment and water inrush mechanism research of deep-buried karst tunnel with rich water.