At present, the study of environmental water demands is focused on the rivers for lack of water resources, but attention is paid little to the rivers of rich water. The paper puts forward and calculates the lowest environmental water demand in river course of stem stream according to the features of ecosystem and environment of lower reach of Yangtze River. The result shows the lowest environmental water demand is about 2 069.2×10 8m3. There is few impact on the ecosystem and environment of the Yangtze Estuary in dry se ason of high flow year, but will cause a serious ecosystem and environment probl ems in dry season of dry year.