The upper reaches of Zhanghe River is a typical water scarcity basin.The rational analysis of water resources system is the premise of optimal allocation of water resources. This paper analyzed the basic characteristics and mechanisms of water resources system of upper reaches of Zhanghe River based on complex adaptive system (CAS) theory and proved that it is a complex adaptive system. Based on this analysis framework,the paper made a deep exploration on evolution road.In the end, the results conclude that as long as government guides water users correctly and water users make active response, the system will inevitably evolve towards the direction of efficient utilization of water resources.