Abstract: The effects of the discharge of the concentrated seawater on the distribution of seawater salinity were investigated using MIKE21 model.With the discharge amounts of the concentrated seawater from Tianjin Dagang and Hebei Huanghua desalination projects as the input conditions, the diffusion of the discharged concentrated seawater in Bohai bay from the desalination projects with different scales during different tide periods was simulated and predicted.The results demonstrated that the dilution of the concentrated seawater was mainly influenced by the ocean currents during different tide periods.Overall, the high water spring tide showed the best dilution and diffusion effects, followed by high water neap tide, the low water neap tide was the worst.The concentrated seawater from the seawater desalination project with the output scale no greater than 10×10⁴ t/d had no significant effect on the sea salinity of the surrounding sea areas.However, the concentrated seawater from the desalination projects with the scales greater than 50×10⁴ t/d could lead to the largest sea area with the increased salinity during low water neat tides.In addition, the sea area with the salinity 1.5 PSU higher than background value and the maximum offshore distance were 31 km² and 6.48 km, respectively.Therefore, the site selection of large-scale desalination project and the discharge of the concentrated seawater should be properly planed to minimize their detrimental effects on the marine environment.