Due to the complex geological conditions of karst area, tunnel construction is often accompanied by potential safety hazard of water inrush from karst caves, especially when the karst cave overlies tunnel (in which condition the water pressure is very important). A numerical model for tunnel under a karst cave is established, and the water pressure of the karst cave is regarded as a construction condition. The variation rules of pore water pressure of surrounding rocks are analyzed, and the distribution of water inrush channel during tunnel excavation is revealed. The water inrush channel and safety thickness of rock under different water pressures of karst cave are decided based on the maximum value estimation method of mutation of the element pore water pressure. The study results show that the distance between water inrush channel and the bottom of karst cave and the safety thickness of rock increase with the water pressure of overlying karst cave increases.