Abstract: This study illustrated the development of eutrophication index over the past four decadesand and collected the application methods about judging eutrophication in China coastal waters in the last 10 years, by calculating the water quality standards of country Ⅰ, Ⅱ, Ⅲ and Ⅳ class water, we found that E maximum threshold were 1.3, 6, 10.7 and 25, respectively;and the reasons of E high values for offshore environmental estuary seawater. The results show that China's coastal waters the values of eutrophication index range was from 0.00 to 137.84; High values of E were not only related to the natural geographical environment of port Bays, but also related to human factors; anthropogenic emissions nutrients of nitrogen and phosphorus were the main factors to cause the seawater pollution.