Abstract: The paper deals with basic thought and method of prediction of water inflow into tunnel by fuzzy clustering theory, and presents main factors infuencing water inflow and their quantized formulae. The water inflow is estimated at 1.0 to 3.0 m³/d·m, taking Qinling extra-long tunnel as an example, which tallies basically with the measured water inflow of 2.07 m³/d·m. This shows the reasonableness of the prediction method.