In order to understand eutrophication and its relationships with environmental factors in gate-controlled river network system of Chongming Island,variations of environmental factors and algae as well as their interaction mechanisms were studied based on seasonal water quality data of the river network.Results indicated that the concentrations of nutrients in the river network were high and nitrogen pollution was particularly serious.The contents of nutrients,Chl.a and trophic state increased significantly after the Yangtze River water entered into the river network.The proportion of diatom decreased with increasing proportions of cyanobacteria and chlorophyta in the river network comparing with the water of the Yangtze River.Although water temperature,pH,turbidity, TN,TP,NO₃^--N and DOC had significant correlations with algal biomass,the relationships between algal community and environmental factors were different.Water temperature and TP were the main factors of promoting growth of cyanobacteria;chlorophyta was closely related to TN and NO₃^--N;and diatom was only related to the turbidity while nutrients were not promoting factors of diatom.