With a hyperbolic natural draft cooling tower of counterflow to be a typical pattern,mathematical and physical models are proposed for fluid flow,heat and mass transfer between air and water inside the tower,in which a "thermal state model" is adopted. The governing equations are discretized in a body-fitted curvilinear near-no nonorthogonal coordinate system and finite-volume method is employed. The SIMPLE algorithm is applied to calculate the flow and temperature field inside the cooling tower. Numerical results are satisfied.