Urban road green spaces constitute a vital element of the urban ecosystem, offering numerous ecological services, including noise reduction, air purification, and the preservation of biodiversity. Spontaneous plants in urban road green spaces are of considerable importance to the urban ecosystem owing to their distinctive natural characteristics. This research centers on a 6 km-wide cross-shaped zone around Hongbo Square in Harbin as the designated study area. Employing systematic and representative sampling techniques, a total of 1 213 sample plots were established to examine the spontaneous vegetation within the herbaceous layer. The study analyzes the species diversity composition, distribution characteristics, and main influencing factors of spontaneous plants across different urbanization levels, road grades, and habitat types. The results indicate that: (1) A total of 296 species of spontaneous herbaceous plants, belonging to 203 genera and 60 families, have been documented. Autochory (self-dispersal) was the dominant seed dispersal mode, while hydrochory (water dispersal) was the least common. (2) The α-diversity in medium and low urbanization areas was significantly higher than in high urbanization areas. The α-diversity levels of urban arterial roads and secondary roads were similar and both significantly higher than that of branch roads. The α-diversity levels of roadside green belts and median green belts were identical and both significantly higher than that of street tree green belts. (3) Seed dispersal modes had a significant impact on the distribution patterns of spontaneous plant diversity: the α-diversity of anemochorous (wind-dispersed) plants varied significantly across different urbanization levels and road grades. In contrast, the α-diversity of autochorous plants differed significantly across different habitat types. The findings of this study aim to provide insights for the low-cost maintenance of urban road green spaces, biodiversity conservation, and urban ecological development. By reasonably preserving and utilizing spontaneous plants, the ecological service functions of urban road green spaces can be maximized.