During tunnel construction in the mountains of southwest China, karst and unfavorable geological conditions often occur. Tunnel construction often encounters unforeseen disasters, such as water inrush, bursting mud, and collapse which cause great hidden troubles to safety. Taking the Wuchiba Tunnel as an example, through applying the supplement geology survey and geology analysis, the karst development was discussed. In the risk evaluation of dangerous parts along the tunnel, the factors such as formation lithology, geological structure, underground water level, and the forecasted discharge were synthetically considered. The risk evaluation system with 4 ranks about karst tunnels was put forth. Based on the risk rank, the advanced forecasting program was designed for the different parts with different ranks, and the disaster prevention measures can be made with the predictable results. The study has great significance for disaster prevention of karst tunnels.