Watershed is the cradle of civilization. Many modern industrial zones are flourishing because they are built near rivers. In the course of human history, however, intensive use has been made of river resources, and the quality of many watershed ecosystem are becoming degraded. Resources are exhausted, environment is seriously polluted, and these degradations have undermined sustainable development of human societies. Watershed is a complex ecosystem with complex interactions between ecology, economy, and society. The research project is designed to study the legal system of ecosystem management for protecting resources and environment at the watershed scale.Watershed ecosystem management is important because it is essential to the preservation of watershed ecosystem. This dissertation analyzes the concept of watershed law in terms of legislative aims, legal concepts, and institutional mechanisms with regard to watershed ecosystem management. The first legislative aim is to protect the resources and environment at the watershed scale, and the ultimate aim is to protect the equilibrium of the whole watershed ecosystem that can sustain harmonious development of ecology, economy, and society. The watershed management law is an integrated area of study, including all ecological, economic and social elements that has a serious effect on watershed ecosystem, and ultimate goal is to bring benefit to the whole watershed system.Considering the trait of watershed ecosystem, the rationale of watershed management law should be mixed and multiple, including bionomics, philosophy, economics, and law. In the fourth part, the dissertation analyzes foreign watershed management laws, such as in America, Australia, Japan, and New Zealand, and compares their differences and sameness. The basic principles of the watershed management law are based on the legislative aim. Thus the dissertation next demonstrates the necessity of obligations in the watershed management system. It then describes the fundamental principles of the watershed