Abstract: The eco-environmental water demand of urban lake include the water for evaporation,itself,habitat,pollutant cleaning,leakage and landscape.Apart from the calculation principles of the priority to protect environment,compat ibility,max and grade,and that timelines should be emphasized.A case study of six lakes of Beijing six lakes is mentioned,and its water demand but winter is calculated.The result is that the minimum water demand in spring is 296.82×10⁴ m³,in summer is 643.94×10⁴ m³,and in fall is 474.59×10⁴ m³,and the all year water demand is 1415.35×10⁴ m³.The percentages of the water demand for pollutant cleaning are 35.87%,71.23%,70.08% and 63.42% separately According to the result,some suggestions about the development of the eco-environment of six lakes are presented.