In order to identified the plant resources and their influencing factors of spatial characteristics of largescale islands, we investigated the Haitan Island, a largescale island in Fujian Province. Combining the line transect method and quadrat sampling method, the plant composition, life forms and floristic geographical components among 20 mountains of this island was investigated. In addition, spearman correlation analysis, variance decomposition and multiple regression were adopted to explain the effects of spatial characteristics on species richness, different groups of plants (life forms, floristic and geographical components of genera) and beta diversity among 20 mountains. The result shows that: (1) there are 541 species of which richness varies greatly belonging to 381 genera and 110 families in 20 mountains of Haitan Island. (2) According to the classification of life forms, nanophanerophytes and therophytes make up the majority of the species. Meanwhile, complex geographical complexity, extensive connections, and strong tropical kinship constituted the characteristics of floristic geography. (3) Elevation and perimeter area ratio are significant factors affecting the species richness and the distribution pattern of plant groups (life forms, floristic geographical of genera) in largescale island mountains. (4) Beta diversity increased with the increase of area ratio and elevation ratio, but decreased with the increase of perimeter area ratio. The study reveals that the plants composition of largescale island mountains is not always affected by area, and elevation and edge effects are also important influencing factors.