Abstract: According to the status quo and developments of coastal power plants，including thermal and nuclear power plants, in China, the quantity of seawater demanded by once-through cooling (OTC) power plants is estimated. Besides, the impacts of impingement with its mechanism and entrainment caused by water intake and warm water discharge on the aquatic organisms and marine ecosystem are discussed. Taken the examples of marine ecosystem changes in Daya Bay before and after the running of power plants, the effects of cooling water intake and discharge are probed. Thus, the hypothesis for the formation of dominant benthic food chain is put forward. Due to the potential and actual negative effects, it is necessary to mitigate the marine environmental impacts of OTC. Based on the site-specific environmental assessment, the best technique available (BTA) shall be adopted for the sustainable use of seawater cooling resource. As a result, the mitigation favors not only for the cool source safety of the power plant but also for the marine environment protections.