Ecological water demand is an important indicator of lake ecosystems, maintaining a virtuous cycle of lake ecosystems. Taking Lake Daihai as the research object, a semi-arid lake in central Inner Mongolia, the dynamic ecological water demand of the lake is analyzed. Based on remote sensing and meteorological data, this research obtains long-term series of high-precision hydrological element data from 1975 to 2020, and analyzes the temporal and spatial evolution of hydrological elements in Lake Daihai; Through the natural ecological water level analysis method, the water level experience frequency analysis method and the lake morphology analysis method, the key water relative depth of the Lake Daihai water level changing with the area is analyzed; Construct a lake ecological water demand model based on the law of ecological water consumption, and calculate the dynamic range of ecological water demand in Lake Daihai under natural conditions; The results of the study are as follows:June to September in Lake Daihai area is the wet season, October to May of the following year is the dry season; Over the past 45 years, the area of Lake Daihai has shown a significant decline, with a rate of decline of 2.3 km²/a, but the rate of decline has slowed in recent years; The suitable ecological water relative depth for Lake Daihai is 8.72-9.92 m in the dry season, and 9.40-10.69 m in the wet season. The suitable ecological water demand is 5.62-7.71 million m³, and the suitable area is 70.92-84.77 km². This paper uses remote sensing technology to obtain hydrological data, build a long-term climatic and hydrological database, and determine the range of dynamic ecological water demand in Lake Daihai can realize real-time monitoring of lake ecological health, and provide scientific basis and operability guidance for related planning and management of Lake Daihai.