<p>The Dongting Lake Basin (DLB) is integral to ensuring ecological security in the middle reaches of the Yangtze River. Alterations in land use and the degradation of ecological functions present significant challenges to the region’s sustainable development. However, existing research has either focused on ecologically fragile upland watersheds such as the Loess Plateau or on highly urbanized regions such as the Yangtze River Delta, leaving a knowledge gap concerning composite basins like the DLB where ecological, agricultural, and urban functions interact dynamically. This study, focusing on the DLB, evaluates ecosystem service functions utilizing the InVEST model. Circuit theory is employed to identify ecological source areas, pinch points, corridors, and barrier points, thereby facilitating the construction of an ecological security pattern. The findings of the study are as follows: (1) Between 2000 and 2020, land use changes in the DLB followed a pattern of two decreases and two increases. (2) During the same period, the water conservation and soil retention functions of the DLB initially declined and subsequently recovered, while carbon sequestration functions were concentrated in the lake area. Urban expansion resulted in localized habitat degradation. (3) The ecological source areas of the DLB in 2000, 2010, and 2020 were identified as 18,925.38 km², 14,770.23 km², and 30,941.38 km², respectively. Ecological corridors numbered 97, 193, and 191; ecological barriers numbered 37, 54, and 45; and ecological bottlenecks numbered 75, 71, and 33. (4) An ecological security pattern was established, characterized by a one core, three zones, three corridors, two axes, and multiple nodes framework. This structure is defined by a central ecological core anchored by DLB, three differentiated management zones for conservation, restoration, and control, three primary ecological corridors to ensure connectivity, two axes delineating urbanization pressures, and multiple critical ecological nodes requiring targeted action. By explicitly linking ecosystem service assessment with circuit-theory-based connectivity analysis, this study advances current ESP research by addressing multi-scale land use conflicts in a composite ecological-agricultural-urban watershed. The results not only demonstrate the remarkable recovery of ecological source areas under national restoration policies, but also provide a novel framework for coupling ecosystem service dynamics with ecological security pattern construction. Thus, this work fills an important knowledge gap and offers a transferable reference for building ecological security in other large river basins.</p>