The ecological water demand by the river system is the least water which maintains the natural ecological structure and the function of the river, having the characteristics of spatially and the timely evolution. It includes the water demand by the watercourse system (including the water transferring sand and the evaporation from water surface) and by the floodplain land. The ecological water demand by the water course system is 44.43 % of average yearly runoff and the ecological water demand by the floodplain land is 1.687 billion being 28.02 % of the total precipitation. The effective countermeasures must be planed to control the exploitation of the water in east Liaohe river basin.