Herbaceous plants that grow naturally in urban monoculture plantations play a crucial role in enhancing plant diversity in cities, while also contributing significantly to the ecosystem services provided by urban greenery. However, current attention to the dynamic changes in the diversity of spontaneous plant communities in urban plantation forests during spring remains somewhat insufficient. This study focused on two common types of urban plantations found in city parks: deciduous broadleaf forests (Populus tomentosa) and evergreen coniferous forests (Pinus tabuliformis). The research examined the variety, abundance, and diversity of spontaneous plants growing under these trees during the spring season. Statistical tools including t-tests, Principal Component Analysis (PCA), and partial Mantel analysis were used to examine changes in understory plant communities over time and their relationship to environmental factors. The results showed that forest stand types significantly affect the composition and dynamic diversity of understory spontaneous vegetation. There were more heliophilous plants in Populus tomentosa forests (6 species) than that in Pinus tabulaeformis forests (1 species), while tolerant plants (15 species) was lower than that in Pinus tabulaeformis forests (21 species). The richness and diversity of spontaneous plants were higher in Populus tomentosa forests than in Pinus tabulaeformis forests.The α diversity of spontaneous plants under Populus tomentosa forests was significantly positively influenced by photosynthetically active radiation and soil temperature (P<0.05), while that under Pinus tabulaeformis forests was significantly positively influenced by soil moisture (P<0.05). Under both forest stands, the β-diversity of spontaneous plants declined significantly, and community composition gradually converged. In addition, the composition of spontaneous plants under Populus tomentosa forests was mainly influenced by diffusion factors, while the composition of spontaneous plants under Pinus tabulaeformis forests was mainly controlled by environmental factors. This study provides a scientific basis for improving plant diversity and green space management under Populus tomentosa and Pinus tabulaeformis forests in cities.