Abstract: How does the long and deep buried diversion tunnel influence the water temperature in an important question related to downstream aquatic organisms.A 3D numerical temperature model for diversion tunnel is established by combining the k-ε model and the energy transport equation.The model is tested with the prototype observed data.The water temperature of the diversion tunnel of Jinping second cascade to be built is predicted and it shows that the increase of the water temperature through the tunnel is not obvious due to the big flow rate,short retention time and large diameter of the tunnel.As the actual full size data is used to build up the computational zone including the concrete lining zone and the action between temperature and velocity field is taken into account,the model can accurately simulate the temperature field and provide scientific reference for the design of hydropower stations and protection of aquatic organisms.