The river-lake-reservoir-pond composite water system in the middle and lower reaches of the Yangtze River represents a critical medium through which watershed hydrological connectivity interacts with ecological processes. However, the coordinated evolutionary dynamics of its multiple components and their underlying driving mechanisms remain inadequately understood. Most existing studies have focused on individual water-body types, which limits a systematic understanding of the dynamic interactions among components within the composite system and the coupled effects of multiple driving factors. Using thematic datasets derived from multi-source remote sensing imagery, this study integrates landscape pattern analysis with the Geographical Detector model to systematically characterize the spatiotemporal differentiation of composite water landscapes in the middle and lower reaches of the Yangtze River. In addition, the interactive effects of natural and anthropogenic driving factors are quantitatively identified. The results show that: (1) During the study period, the total area of composite water landscapes exhibited an overall increasing trend, while their internal structure underwent pronounced differentiation. Areas of rivers and channels, reservoirs and ponds, and shoal lands expanded, whereas lakes, tidal flats, and marsh wetlands continued to decline, highlighting the substantial restructuring of water systems driven by human activities. (2) Significant dynamic transitions occurred among landscape components within the composite water system. Some components experienced net gains, whereas others showed net losses, revealing complex trade-off relationships among water-landscape elements. (3) The overall landscape pattern became increasingly fragmented and complex. Indicators including the number of patches (NP), patch density (PD), edge density (ED), landscape shape index (LSI), Shannon’s diversity index (SHDI), and Shannon’s evenness index (SHEI) increased continuously, whereas the aggregation index (AI) declined. The responses of landscape metrics varied significantly among different water-body types, reflecting divergent evolutionary pathways. (4) The evolution of the composite water landscape was jointly driven by natural conditions and anthropogenic disturbances. Temperature and precipitation established the fundamental regional pattern, while changes in population density exerted particularly strong influences on processes such as lake shrinkage and marsh degradation. Overall, this study elucidates the structural evolution and driving mechanisms of the composite water system in the middle and lower reaches of the Yangtze River, providing a scientific basis for integrated watershed water-resource management, ecological restoration, and territorial spatial planning.