The construction of digital twin watershed has become the core task and goal of the current smart water conservancy construction. Water knowledge graph is one of the most important tools for the construction of knowledge platform in digital twin watershed platform. By structurally and semantically representing knowledge in the field of water resources, it provides important support for decision-making in water resources management, water engineering design, and water environmental protection. This paper aims to review the relevant research of water knowledge graph in order to provide reference for the further development of this field. This paper first introduces the concept and research significance of water conservancy knowledge graph. Secondly, this paper systematically combs the methods and technologies of the construction of water knowledge map, including the key technologies of data acquisition, knowledge extraction, knowledge representation and so on, and it compares and evaluates various methods. Then, from the perspective of hydrology, water resources, water engineering, water environment and other fields, this paper summarizes the existing research achievements of water knowledge graph, and analyzes its value and limitations in practical application. Finally, in view of the existing problems and challenges in the current research of water conservancy knowledge graph, the direction and development trend of future research are proposed, including the deep integration of knowledge graph and machine learning, and the exploration of the application of knowledge graph to water conservancy decision support system, which will help promote the development of knowledge graph research in the field of water conservancy and contribute to the construction of digital twin watershed.