Spontaneous plants are widely distributed in different habitats of urban green spaces and form a diverse species composition, which plays an important role in maintaining and supporting biodiversity. Studying the functional trait composition of spontaneous plants helps to understand how spontaneous plant diversity is formed and maintained in specific habitat. This study investigated the diversity characteristics and functional trait composition of spontaneous plants in eight habitat types, including evergreen and deciduous woodland, evergreen woodland, deciduous woodland, shrubland, grassland, waterside, tree pool and impervious pavement, of urban green spaces in Beijing. The woodland and waterside supported high species richness and functional richness. There were significant differences in the composition of functional traits between habitat types (P ＜ 0.05). The woodlands had a richer distribution of trees and shrubs species as well as alien species, while grasslands were dominated by annual species which were self or animal dispersers. Significant correlations with diversity levels were found for some relatively rare functional traits in spontaneous plant communities (P ＜ 0.05). In shrubland, grassland and waterside, the functional diversity of spontaneous plants increased with the increase in C₄ photosynthetic pathway species. These results can help us to better understand the mechanisms of formation and maintenance of spontaneous plant diversity, and provide a scientific reference for the design of urban green spaces based on the conservation and enhancement of biodiversity.