Abstract: Alien plant invasion seriously threatens the biosecurity and ecological security of islands. In order to explore the invasion status of alien invasive plants in the understory herb layer of Acacia confusa, Celtis sinensis, and Eucalyptus citriodora in Langqi Island and their interspecific relationship with native plants, this study used α diversity, niche, interspecific association, and invasiveness analysis to compare the diversity, niche characteristics, interspecific relationship between alien invasive plants and native plants and the degree of invasion of alien invasive plants in different forests. The results showed that a total of 10 alien invasive herbaceous plants were investigated under 3 forests, belonging to 5 families and 8 genera. Among them, Bidens pilosa has the most considerable niche breadth and essential value, and is widely invaded under 3 forests; the Shannon-Wiener index of exotic invasive herbaceous plants in E. citriodora forest was significantly higher than that in C. sinensis forest. There was no significant difference in Pielou evenness index and Simpson dominance of exotic invasive herbaceous plants under the 3 forests; the niche overlap and interspecific association between most invasive herbaceous plants and native dominant plants in the three forests were low, and the interspecific competition was weak; the invasion intensity coefficient and community invasion coefficient of herb community in C. sinensis and E. citriodora forest were significantly higher than those in A. confusa forest; most alien invasive species in the 3 forests were not competitive with the local dominant plants. Still, the invasion advantage of B. pilosa was enormous, and it was necessary to strengthen the prevention and control. At the same time, protecting the shrub-grass structure and species diversity of the community would help to improve the resistance of the community to alien invasive species.