Abstract: In accordance with the characteristics of water resources utilization in arid zone where a fragile ecological system exitsts and on the basis of theory and method of dual water resources evolution,a mode and calcuation method of rational water resources allocation for the inland arid zone in the Northwest China is formulated,which can maintain the water-land eguilibrium,the water quantiy bulance and the water-salt equilibrium,consider the allocation in space,time,uses,water sources and management,determines the development index by available water resources on the basis of actual situation,and rationally allocate water resources to ecological use and economical use in the unit of a rever basin.The quantitative calculation indicates that among the runoff water resources of 809×10⁸ m³,the water quantity directly allocated to the natural ecological system is 302×10⁸ m³.Plus the returning water of 126×10⁸ m³,the quantity of water which can supply the ecological system amounts to 428×10⁸ m³,and the actual ecological water demand is 385×10⁸ m³,so the water demand of the ecological system can be satisfied on the whole except the Heihe river basin and the Shiyanghe river basin.