As the largest freshwater lake in Yunnan, Dianchi Lake plays a critical role in regional ecological security, with its area, morphology, and surrounding land-use patterns undergoing significant transformations. This study integrates multi-source data—including historical topographic maps, aerial surveys, Keyhole reconnaissance satellite imagery, and Landsat satellite images (1920s–2023)—to construct a century-scale dataset of Dianchi Lake’s area and morphological changes. We analyze the spatiotemporal evolution of its water extent through a dual-method framework: Semi-automatic water reconstruction for the pre-1985 era (lacking satellite imagery) using historical maps and aerial photos; Automated water extraction for 1985–2023 based on Landsat-derived water indices. Key findings reveal: (1) Policy-driven area fluctuations: Water area declined overall from 1920 to 2012 but rebounded after 2012 due to ecological restoration; (2) Morphological complexity: Area decreased by 8.55%, while perimeter increased by 10.44%. The circularity index declined by 26.31%, and the shoreline development coefficient rose by 15.58%; (3) Anthropogenic dominance: Shoreline types shifted from natural-agricultural to artificial, with artificial shorelines increasing 16-fold (1985–2020) and agricultural shorelines decreasing by 44.53%. Extreme drought events accelerated short-term shrinkage.