Abstract: The altitudinal distribution pattern of biodiversity is a hot topic in ecological research. This study took 10 different altitude marsh wetlands in the central and southern part of Xizang Autonomous Region as the research objects to explore the relationship between species diversity, phylogenetic diversity and altitude, as well as the community assembly mechanism. The results showed that: (1) A total of 212 species of seed plants were recorded, belonging to 48 families and 128 genera, with monotypic genera (92 genera, accounting for 71.88%) and perennial herbs as the main components of the community. (2) Species richness, Shannon index and Simpson index were significantly positively correlated with altitude; β diversity was generally low, and 80.67% of the quadrats had Jaccard similarity concentrated in 0.8-1.0. (3) There was no significant linear correlation between phylogenetic diversity index (PD), net relatedness index (NRI) and net nearest taxon index (NTI) and altitude, but both NRI and NTI were greater than 0, indicating that the community phylogenetic structure was aggregated, and the aggregation was more obvious in the high-altitude area. (4) The niche breadth of dominant species varied greatly (1.33-3.91), and 53.3% were dominant species. The seed plant diversity in the Lhasa River Valley showed a unique altitudinal distribution pattern, and environmental filtering dominated the community assembly. The research results provide a scientific basis for the protection of biodiversity in alpine wetlands.