The East China Sea has obvious semi-enclosed sea characteristics according to The United Nations Convention on the Law of the Sea,and the geographical and ecological environment is extremely fragile.With the development and industriali-zation of the East China Sea coast,the degree of marine pollution has gradually increased,making it more difficult to control the pollution and intensifying the destruction of the ecosystem.The existing marine environmental protection cooperation methods,due to their limitations,have limited effect on promoting marine environmental protection in the East China Sea.Such global conventions as The United Nations Convention on the Law of the Sea,as well as the practice of semi-enclosed areas,have provid-ed a legal and practical basis for the construction of a regional legal mechanism for marine environmental protection in the East China Sea.In this context,this paper establishes the fundamental role of the concept of semi-enclosed sea in regional coopera-tion in the East China Sea from the perspective and legal basis of semi-enclosed sea,and discusses the construction of a regional legal mechanism for marine environmental protection in the East China Sea,including the boundaries of cooperation,modes of cooperation,institutional arrangements,and dispute settlement,based on the lessons learned from other legal frameworks for marine cooperation in semi-enclosed sea.