Abstract: By using a improved three dimension numerical ECOM (estuarine, coastal and ocean model), the transport and diffusion of thermal discharge from the Shidongkou electric power plant expansion project in summertime was calculated and analyzed, in terms of the factors of tide, river runoff, wind stress and river surface heat flux. The calculated current speed and direction is consistent with the observed data fairly well, indicating that the model can simulate the hydrodynamic processes correctly. The results show that the thermal discharge is released along with the coast, the influenced extension in the downstream is larger than that in the upstream due to the effect of river runoff. During spring and neap tides the timeaveraged temperature rise near the waterspout of the project is 2.34 and 2.84 ℃, respectively; the areas with surface temperature rise 1.0 ℃ is 0.09 and 0.20 km2, respectively; and the areas with bottom temperature rise 1.0 ℃ are both 0.09 km2, respectively. The current speed is larger, the advection and lateral diffusion of water temperature is greater in spring tide, resulting in temperature rise near the waterspout of the project and its expand extent and magnitude along the coast is obvious smaller in spring tide than in neap tide.