Aiming to understand the response of urban semi-natural plant communities to environmental factors and the mechanism of maintaining functionality of urban semi-natural plant communities. In this study, we focused on semi-natural plant communities in Fuzhou city. Based on the investigation of plant communities, we used correlation analysis, redundancy analysis and structural equation model to explore the relationship between leaf functional traits (leaf length-to-width ratio, leaf area, leaf thickness, specific leaf area, leaf tissue density, leaf dry matter content, leaf carbon contents, leaf nitrogen content, leaf phosphorus content) at the community level, species diversity (Shannon-Wiener index, Pielou evenness index, Simpson index, Margalef index) and functional diversity (functional richness, functional evenness, functional dispersion, Rao’s quadratic entropy). The results were as follows: (1) The species diversity of the shrub layer and the herb layer in the semi-natural plant community in Fuzhou city was significantly higher than that in the arbor layer, with invasive plants appearing in the herb layer; (2) There was a strong correlation between leaf functional traits and plant diversity, with a negative correlation between functional richness and species diversity; (3) The soil temperature and total nitrogen content provided a strong explanation for the functional traits and plant diversity of urban semi-natural plant communities, and the ability of plants to absorb soil nutrients was influenced by temperature and water; (4) Structural equation model showed that atmospheric temperature and leaf dry matter content have a direct and significant impact on functional richness, while soil temperature and Pielou evenness index have an indirect impact on functional richness through leaf dry matter content. In summary, leaf functional traits and plant diversity in urban semi-natural plant communities are influenced by both biological and abiotic factors.