The extremely low water level of the Dongting Lake during the dry season has a serious impact on the ecological environment of the Lake. The Three Gorges Reservoir (TGR) has increased the discharge for the lake to supplement water during the dry seasons. In order to quantitatively describe the spatial difference of water supplement effect on the ecological water supplement from the operation schemes of TGR at dry seasons, three typical schemes were proposed, namely, drop in advance, uniformly drop, and drop at high water level. Based on the integrated couplling mathematical model of rivers and lakes, the spatial distribution of Dongting Lake water level under different schemes was analyzed, and the impact of different schemes on power generation was evaluated. The results showed that the influence of drop in advance and drop at high water level on the water level of Dongting Lake had obvious spatial heterogeneity, and the influence on the northern part of East Dongting Lake was obvious, whereas for the south and west Dongting lake was small. The water level of Chenglingji increased 0.12 m on average, and the power generation of Three Gorges power reduced 0.30% for the drop in advance, while for the drop at high water level the water level of Chenglingji reduced 0.09 m on average, and the power generation increased 0.28%. The research results could provide beneficial reference for the formulation of the Three Gorges Reservoir's drop mode taking into account the ecological water supply of the Dongting Lake.