Abstract: In recent years, the dry-season hydrological regime of Poyang Lake has undergone significant changes, posing serious threats to the water supply security of surrounding regions. Based on water level observation data from major hydrological stations of Poyang Lake, runoff data from the five rivers, and Landsat satellite images during 1980-2022, this study employed methods such as Mann-Kendall trend analysis, mutation point identification, remote sensing water body retrieval, and spatial analysis to systematically examine the temporal duration, water level variations, and spatial differentiation characteristics of dry seasons before and after the operation of the Three Gorges Project. The operational efficiency of water intake facilities and the water source structure under different water level conditions were quantitatively evaluated. The results indicate that after the official operation of the Three Gorges Project, the onset of the dry season in Poyang Lake advanced by more than 35 days on average, while the end was delayed by over 20 days, leading to an overall extension of the dry season by 58-83.3 days and an average water level decline of 0.61-1.76 m. Among the monitoring stations, the water level decrease was most pronounced at Duchang Station, while Kangshan Station remained relatively stable. The Ganjiang River constituted the primary water source, accounting for nearly 60%-71% of the total inflow, whereas the Raohe River contributed less than 6.7%, with a sharp reduction under extremely low water level conditions. A water level of 12 m at Xingzi Station was identified as the critical threshold for regional water supply security. Below this level, the reliability of water intake significantly decreased, potentially affecting up to a million people. By integrating multi-source data and diverse analytical methods, this study reveals the evolution mechanism of dry-season water levels and the corresponding water supply response under changing river-lake interactions, identifies key water level thresholds and their spatial heterogeneity, and provides a scientific basis and decision-making support for collaborative water resource management, adaptive improvement of water supply projects, and risk prevention in the Poyang Lake basin.