Based on triangular grid, a 2D unsteady flow mathematical model for warm water drainage in a power plant has been established with finite element method. Since the model is built on the unsteady hydrodynamics, the model can be used in all kinds of steady and unsteady river flow and suitable for several drainage styles. Now the model has been applied successfully in many power plants. A simulation for Madu Power Plant in Nanjing is given in the article, and the calculated results are compared with measured results of the physical model with the warm drainage. At last, the dynamic displaying of flow field and temperature rise field is illustrated in the paper.