Species and functional diversity of spontaneous herb communities directly affect the functionality and stability of urban ecosystems, but relevant studies are still limited. In this paper, taking Shenzhen City as a case study, we analyzed the community structure, species diversity, functional traits, and functional diversity of spontaneous herbs based on the field survey data of 600 sample plots with 1 hm² sample plots in the built-up area. The results showed that: ① A total of 273 species, 178 genera, and 61 families of spontaneous herbs were recorded in the survey, with perennial and native herbs accounting for 54.6% and 65.6%, respectively. ② Predominant functional traits were small and medium-sized leaves (91.85%), herbaceous leaves (42.59%), papery leaves (32.96%), glabrous or subglabrous leaves (56.67%), moderately densely tomentose (40.37%), rougher foliage surface (52.59%), flowering period of 4-6 months (50.78%), fruiting period of 4-6 months (49.22%), and dry fruits (86.03%). ③ The differences among plant communities were extremely significant for functional richness FDp, functional dispersion Rao quadratic entropy (P ＜ 0.01), and significant for functional evenness FEve (P ＜ 0.05). ④ The species diversity index of park green space was significantly higher than the other three types of green space. The functional richness and functional dispersion had extremely significant differences among green space types in dust retention, noise reduction, temperature and humidity reduction, and biodiversity conservation (P ＜ 0.01). The difference in functional homogeneity was highly significant only in dust retention and biodiversity conservation (P ＜ 0.01). ⑤ All functional diversity indices showed highly significantly positive correlations with the Margalef, Simpson, and Shannon-Wiener indexes (P ＜ 0.01). The conclusions and methods of the study provide a theoretical basis for maintaining the stability of urban ecosystems, building ecologically livable cities, and promoting sustainable urban development.