Abstract: On the base of redefining usable water resources and ecological water demand the water resources carrying capacity of Northwest China is estimated on the scale of basins.The ecological water demand and water rights was neglected in the former definition of usable water resources.This paper defines usable water resources for human being as:One part of total available water resources that can be consumed by human living and economic production under conditions of certain techno-economic level,certain standards of ecological protection and water right limitation.Taking the viewpoint of runoff resources,ecological water demand means one part of total available water resources that is needed to maintain the integrity of ecosystem.The total available water resources is balanced by human usable water resources,ecological water requirement and water resources not usable by human being and ecosystem we want to protect.The total renewable water resources of Northwset China (including internal closed basins and Yellow River basin within Shaanxi,Ningxia,Gansu,Qinghai,Xinjiang and the westem part of Inner Mongoulia) is 1 638 1 5×10⁸ m³,while the net amount that flow into other countries is 411.9×10⁸ m³,the quantity if ecological water requirement is 454.4×10⁸ m³,and the quantity of usable water resources is 742×10⁸ m³.By using an optimizing model the water carrying capacity of Northwest China is appraised:under the conditions that per capita GDP double every 10 years and water use efficiency rise at the rats of 7% annually,water resources of Northwest China can support population of 12.7 million persons in year 2030.