Abstract: In order to understand the effect of the freshwater letdown on the seawater desulfurization of the power plant, a hydrodynamic and transport model of Mike 21 were adopted to simulate tidal flow field and convection diffusion filed.The convection diffusion of HCO₃^- of spring tide and neap tide was simulated for 12 months, according to the upstream runoff rate measured data.The simulation results show that, the effect of freshwater letdown is bigger during July to September, the concentration of HCO₃^- is less than 90 mg/L, and the minimum value is 78.32 mg/L.In the other months, the concentration is about 95 mg/L.According to the results above, it could ensure the desulfurization efficiency of electric field seawater and control the construction cost, by changing the number of desulfurization pumps and selecting different sulfur-containing coals.