Eucalypt plantations expand rapidly out of their natural distribution zones, thus inducing a concern on their effects on biodiversity and ecosystem functions. We compare the understory plant diversity of 46 plots of eucalypt plantations, including early and later stages in rotation, with that of 21 plots of contrast vegetation, including other types of plantations and secondary shrub grassland, in Guangdong and Guangxi Provinces, South China. Although the overall plant diversity did not change significantly in eucalypt plantations relative to the contrast vegetation, the community structures changed dramatically. The Asteraceae family, which is the most important source of destructive invasive plant species in China, is ranked 3(rd) (7.42%) and 7(th) (3.14%) in species importance in the early and later stages in eucalypt plantations, respectively. Nevertheless, Asteraceae is ranked 15(th) (1.73%) in other types of plantations and 21(st) (0.94%) in secondary shrub grassland. Significant increases in the richness and frequency of invasive species were also observed in eucalypt plantations. Among the 20 invasive species recorded in the eucalypt plantations, 9 species were destructive invasive species and 7 of these species belonged to Asteraceae. This study highlights an enhanced plant invasion risk in eucalypt plantations in South China, particularly by Asteraceae.