Abstract: Thewater quality of a northern large reservoir was studied and analyzed based on the average physical and chemical values of the water in recent years from the representative water quality monitoring stations of the reservoir. The system analysis of water temperature,pH,nutrient and heavy metal and toxic substances,and etc.was conducted on the basis of the comprehensive evaluation of average water quality. The water pollution situation of the representative monitoring points was examined as well as the organic pollution. The countermeasures were put forward.