Abstract: In this study, site surveys and environmental factor analyses were carried out on Eucalyptus plantations in Guilin, Nanning, and Beihai in Guangxi. Species and phylogenetic α- and β-diversity patterns of the shrub and herbaceous layers of the Eucalyptus plantation understories and their influencing factors were analyzed in the three regions. Results showed that species α-diversity of the shrub and herbaceous layers in the Eucalyptus plantations tended to increase from Guilin, Nanning, to Beihai. Species α-diversity indices of the shrub layer, including Shannon-Wiener, Simpson, and Patrick richness indices, were significantly smaller in Guilin than in Beihai, while the Pielou evenness index of the herb layer was significantly smaller in Guilin than in Beihai. Species β-diversity analysis showed that the shrub and herb layer species between Nanning and Beihai were the most similar, while herb layer species composition between Guilin and Beihai was the most different. Further analysis showed that phylogenetic α-diversity of the shrub layers gradually increased, while that of the herb layers first decreased and then increased. The mean phylogenetic distance (MPD) and mean nearest taxon distance (MNTD) of the shrub layer and the MNTD of the herb layer showed a single peak pattern. Redundancy analysis indicated that species and phylogenetic diversities of the Eucalyptus plantation understories in Guangxi were mainly influenced by slope degree, soil total phosphorus, and elevation. These results help to reveal the patterns of plant diversity and influencing factors of Eucalyptus plantations in Guangxi, providing a reference for scientific research and management.