In order to clarify the impact of island spatial characteristics on plant species diversity, this study applied the sample line method and sampling method to conduct vegetation survey on 19 uninhabited islands around the main island of Pingtan, Fujian Province, and statistically analyzed the species richness of each island and the Margalef richness index, Shannon-Wiener diversity index, Simpson diversity index and Pielou evenness index of typical plant communities. We used multiple functions to fit the species-area relationship, and used Redundancy analysis to explore the spatial factors that affect the species diversity of typical plant communities on the island, so as to explore the formation and maintenance mechanism of plant species diversity on the islands. The results show that: (1) among the 19 uninhabited islands around the main island of Pingtan, 12 islands are dominated by typical island forest communities, 2 islands are dominated by shrub and grass layers and lack of tree layer, and 5 islands are only covered by grass. (2) The plant species richness of the 19 islands increased with the increase of the island area. When the island area increased to 7.184 hm², the rate of species increase slowed down. (3) The area, perimeter, perimeter area ratio and inshore distance of the island are the main spatial characteristics that affect species diversity. The larger the area and perimeter of the island, the smaller the perimeter area ratio and inshore distance, the higher the species richness of the island. (4) In the typical plant communities of the island, the species diversity of the tree layer is mainly affected by the island area, perimeter and perimeter area ratio, while the species diversity of the shrub and grass layers is mainly affected by the inshore distance. According to the research, the island area of 7.184 hm² can guarantee the survival space of most common plant species. The closer the island is to the mainland, the more conducive it is to the spread of species, and the higher its species immigration rate, thus improving the islands species diversity.