Abstract: According to the investigations of four mid-months (Feb.,May,Aug.,and Nov.)of seasons in 2004,we choose COD,DO,PO₄-P,DIN,Chl-a as assessing indexes,and use the artificial neural network to assess eutrophication in the Yangtze River estuary and its adjacent waters.The results show that the waters of eutrophication mainly concentrate round the rivermo uth,and the extent of eutrophication gradually decreases from the rivermouth to the east and northeast.The areas of eutrophication are almost distributed in the side of salinity bellowed 20 and have seasonal variations along with the changes of Yangtze River diluted water.The phenomenon of eutrophication is more serious in May and August than in other months,and both have the trend of turning around to the northeast,which may be caused by the diluted water turnaround with the flood coming in May.The transection distributions of eutrophication in each rmnth indicate that the grades of eutrophication change from high to low level,from rivermouth to the east,and it has complex variations in the vertical direction.The analysis shows that DIN is the main control factor for the assessment results of eutrophication.