Daihai Basin is located in Ulanchabu City, Inner Mongolia Autonomous Region, China. It is surrounded by mountains, which is a typical closed inland brackish lake. Daihai Basin is in the transition zone from temperate semi-arid zone to arid zone. In the past 30 years, the area of Daihai Lake has shrunk sharply and CODMn has remained high due to the influence of human activities and natural factors. The project collects and collates relevant historical data and scientific research achievements of water environment, water resources, social economy and land use in Daihai Lake basin; The seasonal changes of water quality, sediment， ecology and the sources of surface runoff pollution were investigated; The population, social economy, pollution, source of load, land use status etc were investigated. The source-sink relationship of Daihai Lake, water resources balance analysis and CODMn were Analyzed. On this basis, the range of environmental quality baseline values of CODMn in Daihai Lake under non-human activities is analyzed, and the phased objectives of its protection are put forward.