Jinji Lake is located in the developed area of Suzhou City, Jiangsu Province.The area of Jinji Lake is about 7.38 km², with mean water depth of about 1.6 m and a volume of about 0.12×10⁸ m³.According to National Environmental Quality Standards for Surface Water of P.R.China, the water quality in Jinji Lake is about Grade IV-V which is mainly caused by the industrial, agricultural and sewage pollution around Jinji Lake.The municipal government of Suzhou City decides that Jinji Lake will become a lake for of sight seeing near the city area, so that the function of the lake will be changed from irrigation to the entertainment.In order to improve the water quality in Jinji Lake, the comprehensive measures is planned to putinto the use in Jinji Lake.The environment benefit and economic expenses of the engineering to improving the water quality in Jinji Lake have been analyzed comprehensively in this paper.The engineering can change the water quality in Jinji Lake from Grade IV-V to Grade III (National Environmental Quality Standard for Surface Water, PRC).As cauculated, the expected economic expenses is about 0.92 to 1.62 Yuan RMB/m³.If we consider the rise of landed estate and travel income once the lake water quality has been improved, the economic expenses would be lower.In conclusion, the engineering could bring out obvious environment and social benefits, and the economic expenses is also reasonable.