Abstract: Water resources system, which mainly deals with those problems both directly and indirectly related to utilization and protection of water resources and water emvironment, is a specialized kind of natural-social-economic complex system. In this paper, the authors concentrate upon the topic of its fundamental characteristics. First, these relationships between water and natural ecosystems, and water and social developments are discussed. Then, the role of water-work in coordinating natore and social developments is analysed. The authors point out that human water-work is a unity of numerous natural and social processes. Fnally, the general characteristics of complex water resources system as a whole are briefly outlined.