Abstract: During the construction of tunnels in karst mountain, water inrush and gushing mud (sand) disaster has ranked the first in all the disasters. The exact and timely geological prediction is the critical problem most needed to be studied and solved in the design and construction of tunnels in karst zone. The theories of geology and karstology are used for the geological analysis of the tunnel. A fuzzy mathematical model is put forward to evaluate the risk of different parts along the tunnel, and the risk rank is given. According to the advantages and disadvantages of various geophysical methods and the sensitivity of forecasting karst water, and with the method and principle of the comprehensive geological prediction, the forecast program and process of comprehensive geological prediction are optimized and the accuracy of the position exploration of karst water is improved greatly. And the forecast technology is used during the construction of the Wuchiba tunnel of Shanghai-Chendu Expressway west. During the construction of the tunnel, the comprehensive geological prediction technology is implemented strictly. Through analyzing the geology and the fuzzy mathematical model, different risk rating parts of geological disasters along the tunnel are divided. In different risk rating zones, different programs are enacted. Based on the programs, the karst water is successfully forecast, and the scientificity and feasibility are validated.