The spatial distribution patterns, phenotypic responses, and environmental driving mechanisms of dominant plants in the riparian zone of large reservoirs are key to understanding plant adaptation mechanisms and the community succession to the alternating terrestrial and aquatic habitats. This study focuses on Cynodon dactylon (L.) Pers.), a typical dominant plant in the drawdown zone of the Three Gorges Reservoir.. Through comprehensive vegetation and soil surveys, and using methods such as spatial interpolation, analysis of variance, and regression analysis, we analyzed its spatial distribution, phenotypic variation, and environmental drivers. The results showed that the coverage of C. dactylon exhibited a horizontal spatial pattern, characterized as high in the mid-section yet low in both the upper and lower sections of the reservoir. with its coverage and density in the mainstream significantly lower than in the tributaries. Along the elevation gradient, both the coverage and density of C. dactylon in the 165–175 m elevation zone were significantly lower. Analysis of environmental factors revealed that the coverage and density were primarily regulated by soil bulk density and phosphorus content, while high nitrogen environments drove preferential biomass investment into leaves to enhance photosynthetic capacity. The study demonstrates that C. dactylon adapts to environmental heterogeneity in the hydro-fluctuation zone by adjusting biomass allocation strategies and exhibiting phenotypic plasticity, providing a theoretical basis for near-natural restoration of reservoir riparian zones.