To elucidate the response and adaptation mechanisms of forest plant communities to urbanization, this study selected five forest parks along an urbanization gradient in Hangzhou as research subjects. Through plant line transect/quadrat surveys, combined with diversity indices and Levins' niche breadth, and employing multivariate statistical analysis, we systematically investigated the response patterns of plant community composition, diversity, and niche characteristics to urbanization. The results indicated that: (1) The response of plant diversity to urbanization is complex: parks with lower urbanization levels exhibited higher species richness, whereas those with higher urbanization levels showed greater species evenness and higher Shannon diversity indices, with significant differences in species composition among parks (mean Jaccard dissimilarity: 0.58). (2) Key species displayed differentiated ecological adaptation strategies: shared tree species such as Castanopsis sclerophylla and Liquidambar formosana functioned as broad-niche competitors in low-urbanization environments but shifted to narrow-niche dominant species in high-urbanization environments, demonstrating a "niche shift" strategy and potential as key species for restoration. Species like Litsea cubeba, whose importance values declined with increasing urbanization, can serve as indicators of habitat quality. (3) Functional reorganization occurred in the adaptive structure of plant communities: the proportion of specialists increased continuously with urbanization (from 57.73% to 65.45%), becoming the dominant group; the proportion of intermediate species declined monotonically (from 32.99% to 27.27%), supporting the "decline of moderate adapters" hypothesis; generalists were significantly suppressed under extreme urban environments (proportion decreased to 7.27%). This reveals that urbanization acts as a "specialization filter", driving the functional trait spectrum of communities toward a specialist-dominated structural reorganization by eliminating intermediate species, suppressing generalists, and aggregating various specialists. (4) Anthropogenic disturbances have become the dominant driving factor: the importance value and niche breadth of shared species were positively correlated with road density and population density but negatively correlated with natural topographic factors, indicating that human-mediated selection mechanisms have gradually replaced the dominant role of natural topography in the context of urbanization. This study demonstrates that urbanization reshapes the plant composition of urban forests by filtering sensitive species, releasing resistant species, and promoting the systematic reorganization of community structure and functional traits. Therefore, in urban ecological restoration, priority should be given to native key species with "urban adaptability" to facilitate the establishment of near-natural, highly resilient forest communities.