Abstract: By means of trophic state index(TSI)and its modificator(TSIm)in which algal chlorophyll a(Chla), water transparency(Sd)and total phosphorus(TP)are used as evaluation factors, the continuous numerical classification for the trophic states of three waterbodies(Dongtang, Wuhu and Xiantang reservoirs)of 1998 and 1999 in Yujiang county, Jiangxi province were obtained. The results indicated that all the three reservoirs were in the state of eutrophication. The values of TSI and TSIm of Dongtang reservoir were 69 and 70 respectively, which means that the trophic state of this reservoir was highly eutrophicated and was the worst among the three reservoirs. Although Wuhu reseroirs got the lowest values of TSI and TSIm, they also reached 60 respectively. Thereafter principal component analysis(PCA)was carried out to explore which factors(including K++Na+、PO4-P、TP、NO2-N、NO3-N、NH4-N、TN、SiO2、pH、COD)were specially responsible to the water qualities of these waterbodies. The results showed that NH4-N was the most important factor to the first principal component, which weighted 0.986, and NO2-N was the second weighted one with the corresponding value of 0.909, so it can be inferred that nitrogen was the main contributor to the first principal component. With the loading of 0.837, COD dominated the second principal component. Studies on the data of September of 1999 showed that COD acted as the most important role in the water trophic state of three reservoirs. Corresponding analysis suggested that it was local agricultural system that resulted in superfluous organic fertilizers, which had been poured in these reservoirs and eutrophicated the waters. This result could be important to local agriculture and reservoir managements.