Abstract: The thermal buoyancy effect of the cooling water is clear in deeper water areas because the heat quantity of the cooling water is diffused at certain depth below water surface. The diffusion effects of two types of diffusion depth of cooling water, such as the whole water depth and the mixing zone, are calculated by using a twodimensional mathematical model. Comparison of the calculation results shows that it is more rational using the mixing zone depth to control the cooling water diffusion for deeper water.