Abstract: As an ecotone between closed-canopy forest and alpine vegetation, the treeline serves as an early-warning indicator for global climate change. This study focuses on treeline forest communities of Li Mountains, Shanxi Province. A method combining field surveys (32 plots of 10 m×10 m) and laboratory experimental analysis was adopted to explore plant biodiversity patterns and the relationship between community composition and environmental factors. A total of 80 plant species belonging to 34 families were recorded, with Rosaceae and Asteraceae identified as the dominant families. Key findings included: Distribution patterns of species diversity, functional diversity, and phylogenetic diversity exhibited differences, with overall integrated diversity being relatively high. The communities displayed low species evenness but high functional evenness, with weak functional divergence (RaoQ) and phylogenetic divergence (NRI). This suggested that habitat filtering dominated community assembly, reflecting a trade-off between quantitative competition and functional complementarity among species. Significant positive correlations were found among the three types of diversity indices. Specifically, species richness showed significant positive correlations with functional richness (FRic) and phylogenetic richness (PD), and FRic and PD were also significantly positively correlated. Community composition and environmental factors analysis indicated that slope aspect, soil depth, litter thickness, soil total nitrogen, total sulfur, and organic carbon were key drivers of the composition and structure of forest communities along the treeline of Li Mountains. This study identifies the key factors influencing plant diversity in the treeline of Li Mountains, providing crucial support for regional ecological restoration, optimal vegetation configuration, and the formulation of biodiversity conservation strategies.