A 3-D temperature model for diversion tunnel was developed by combining k-ε model and energy transport equation. The actual geometry data of the tunnel was used to construct a three-dimensional computational domain including the concrete lining. As the interaction between temperature and velocity fields can be taken into account, the model can be employed to accurately simulate the temperature and velocity field. The model was validated with the field data observed in the diversion tunnel of the Nanya River. The water temperature of the diversion tunnel of the Second Jinping Cascade Hydropower Station to be built was predicted. It is shown that the temperature increase is not observable due to the large diameter of the tunnel, the big flow rate and the short contact time. The result can provide scientific foundation or reference for the design of hydropower station, and protection strategy of aquatic organisms and aquatic ecosystem.