Eutrophication is impairing the function of fresh water lakes and reservoirs witch are the main water source for agriculture irrigation, flood control , power generation and domestic water. The paper took the ten indexes (UV254 and CODMn, TOC, soluble silica, total nitrogen, nitrate nitrogen, ammonia nitrogen, nitrite nitrogen, total phosphorus and phosphate) as independent variables,and chlorophyll a as attributive variable during dynamic monitoring of a water source reservoir in south China. Through the methods of principal component analysis (PCA) and multiple linear regression (MLR),the paper got the fitting curve of taking TOC,total nitrogen,total phosphorus and nitrite nitrogen as independent variables and chlorophyll as attributive variable. Through the test of F and Sig value, F and Sig value are 18.670and 0respectively, which indicates a very good linear correlation and confirms good accuracy. The validation test of the model shows 86.1% fitness, which demonstrates the forecasting ability of the model. chlorophyll a and total nitrogen (and/or total phosphorus) are demonstrated to be the eutrophic control factors of the study reservoir.The total nitrogen and total phosphorus have the similar influence on europhication level of water reservoir.