Abstract: Taking the surrounding areas of Xiaowan Reservoir in the middle reach of Lancangjiang River as study area, and based on the vegetation investigation at three sites including electricity transmission area (site 1), electricitytransfer substation and roadsides to the substation (site 2), and emigration area (site 3) in 1997 (before dam construction), another investigation was conducted on the vegetation composition, plant coverage, and dominant species at the same sites in 2010 (after dam construction), aimed to evaluate the ecological risk of the dam construction for the terrestrial plant species in middle reach of Lancangjiang River. There was an obvious difference in the summed dominance ratio of dominant species at the three sites before and after the dam construction. According the types of species (dominant and non-dominant species) and the changes of plant dominance, the ecological risk (ER) for the plant species was categorized into 0 to IV, i.e., no or extremely low ecological risk (0), low ecological risk (Ⅰ), medium ecological risk (Ⅱ), high ecological risk (Ⅲ), and extremely high ecological risk (Ⅳ). As affected by the dam construction, the majority of the species were at ER Ⅲ, and a few species were at ER Ⅳ. The percentage of the plant species at ER Ⅲ and ER Ⅳ at site  3 was higher than that at sites  1 and  2. The decrease or loss of native plants and the increase of alien or invasive plants were the major ecological risks caused by the dam construction. Effective protection strategies should be adopted to mitigate the ecological risk of the dam construction for the terrestrial plants at species level.