Abstract: In order to study the changes in water storage capacity and the degree of ecological water demand guarantee in Dongting Lake, this paper uses 231 satellite remote sensing images since 1994, combined with hydrological data such as water level, runoff, and precipitation, to fit a model of the relationship between water surface, lake capacity, and water level in Dongting Lake in the past 30 years, revealing the temporal changes in water storage capacity of Dongting Lake before and after the operation of the Three Gorges Reservoir (TGD). The "annual distribution method of ecological water demand" is adopted to analyze the degree of ecological water demand guarantee in Dongting Lake and analyze the driving reasons. The results show that: (1) In the past 30 years, the average annual water storage capacity of Dongting Lake has generally decreased, from 5.141 billion m³ before TGD operation to 4.583 billion m³ after operation, a decrease of 10.85%. The largest decrease was in October, reaching 24.50%, which coincides with the time when TGD controls its discharge due to power generation in October each year. (2) After the operation of TGD, the ecological security of Dongting Lake water has become increasingly severe, with the annual minimum and suitable ecological water demand guarantee degree and maximum ecological water demand exceeding degree decreasing from 96.38%, 68.05%, and 20.69% to 90.30%, 62.71%, and 16.84%, respectively. The minimum and suitable ecological water demand guarantee in September and October has significantly decreased, exacerbating the autumn water shortage and drought situation in Dongting Lake. The maximum ecological water demand in July and August has significantly decreased, alleviating the hidden dangers of flood disasters in Dongting Lake during the flood season. (3) The decrease in runoff, average annual precipitation, and continuous sediment deposition in Dongting Lake are the main drivers of the reduction in water storage capacity and security. Sand mining has expanded the lake capacity of Dongting Lake, but the increase in water storage capacity is extremely limited. Satellite remote sensing technology provides a new technical means for studying the degree of water storage guarantee in lakes, which has practical significance for supporting the allocation of water resources and water ecological restoration in Dongting Lake.