Based on the definition and characteristics of Complexity, it's revealed that the water resources system, like Complexity, has the subject-object essence. The water resources system in terms of economy, environment, humanity, water planning was analyzed. It is regarded as an hierarchical network consisting independent adaptive individuals. Regularity of the system's evolution was obtained through quantitative and qualitative approaches. To establish a complex adaptive system (CAS) of water resources in a river-basin-level will directly promote water resources management and eco-balance. 