Abstract: The changes of aquatic microbial communities in a continuous water body on Tsinghua campus were investigated by polyurethane foam unit method.The measured composition of the communities included bacteria,fungi,algae,protozoa and some metazoa(such as rotifer).The measured indicators of water quality included water temperature,pH value,DO,COD_Mn ,TN,TP and chlorophyll a.The degree of eutrophication was also assessed with AHP-PCA method,the TLI_c s were computed out for each sampling site.It was discovered that there are high correlations between aquatic microbial community parameters (such as S_ep , G,T_90%and HI) and the indicators of water quality (such as TLI_c).