Lakes, as an important type of water bodies, play indispensable roles in water regulation and storage, water ecological environmental protection and regional climate regulation. Many countries in the world are still faced with severe challenges of environmental sustainability in lake management, water resources use, and eco-environmental protection. The ecological pattern of lakes in China has also undergone new changes, such as lake water depletion, water ecology deterioration and regional climate revolution, which have brought new issues and challenges to the flood control safety and ecological security of lakes. River-connecting lakes are common all over the world, which are generally connected with big rivers to form water system. The regulation and storage of lake can impose significant effects on the inflow and the outflow between river and lake. The hydrological factors are important indexes for characterizing the change of lake. Analyzing the lake’s hydrological variation characteristics can directly reflect the changes of the lake in water level and volume. Accordingly, this study focused on the Lake Ulansuhai in Inner Mongolia, China, and analyzed the variation characteristics of water level and outflow over the past years. The water level of the Lake showed significant increasing trend and may keep increasing in future from 1978 to 2019. The multi-year average annual outflow during the period from 1977 to 2017 was 8333.7 million m3. On account of artificial regulation, the outflow from the total drainage to the Lake overall increased, without obvious regularity.