Riparian vegetation performs important ecological functions in protecting water quality, stabilizing the bank, maintaining habitats connectivity and enhancing local biodiversity. Yet, riparian vegetation has been degraded severely throughout the world mainly due to anthropogenic disturbances. This study focused on the riparian vegetation communities and the impact of environmental factors on spatial distribution of plant communities in the riparian zone of Danjiangkou Reservoir, the water source for the Middle-Route of the South-to-North Water Transfer Project. Field survey was carried out from April to May, 2011. The 201 plant communities were investigated. And three topographic factors (elevation, slope and aspect), eight soil factors (soil thickness, soil texture, soil pH, organic matter, total nitrogen, total phosphorus, available nitrogen, and available phosphorus), and two hydrological factors (flooding duration and flooding frequency) were measured. On the basis of the investigation of riparian plant communities and environmental factors, the 201 plant communities were classified into different community types by means of Two Way Indicator Species Analysis (TWINSPAN); the impact of environmental factors on riparian plant communities were analyzed by using Detrended Canonical Correspondence Analysis (DCCA) combined with topographic, hydrologic and soil factors. And the explanatory power of environmental factors on the riparian plant community was quantitatively partitioned. The results showed that (1) the riparian plant communities were classified into seven types of Polygonum aviculare community, Abutilon theophrasti community, Oenanthe dielsii var. stenophylla + Cynodon dactylon community, Cynodon dactylon community, Populus adenopoda-Cynodon dactylon community, Pyrus betulifolia-Sophora davidii-Cynodon dactylon community and Platycladus orientalis-Vitex negundo var. cannabifolia-Carex tristachya community; (2) the results of DCCA indicated that riparian vegetation and environmental factors had a significant correlation. The first axis had a significant correlation with elevation, flood duration and flood frequency, it indicated that elevation and flooding played a dominant role in spatial distribution of the riparian plant communities. Soil organic matter content, pH and total nitrogen content presented a significant correlation with the second axis, which indicated that soil nutrient also played a role in spatial distribution of the riparian plant communities. (3) the effects of soil factors, hydrology factors, topographic factors and their interaction on the total variation of riparian vegetation were quantitatively partitioned and showed that the contribution rate of the soil factors was 10.24% separately, and the contribution rate of the topographic factors was 3.15% and the hydrological factors was 3.04% separately. 0.73% for soil factors coupled with topographic factors, 1.22% for topographic factors and hydrological factors, 0.75% for hydrology factors with soil factors, and the interactive effect of topographic, soil and hydrological factors was 2.86%. The collected environmental factors accounted for 21.99% of spatial distribution of the riparian plant communities, the unexplained portion for 78.01%. Our results demonstrated that the explanatory power of environmental factors was determined by the complexity of vegetation, and the more complexity of vegetation, the less explanatory power by environmental factors. The riparian zone is a typical ecotone, and environment and vegetation are complicated and varied. These caused that the explanatory power of environmental factors was low. The unexplained portion may be caused by climate, biotic interaction, interference and random factors.