In order to explore the distribution pattern of phytoplankton community and the temporal and spatial niche characteristics of dominant species in the middle reaches of Yarlung Zangbo River, phytoplankton samples were collected and physical and chemical factors were measured in July and October 2021 to identify phytoplankton species. The niche width, niche overlap value, ecological response rate, and relative resource share of dominant species were calculated. The cooccurrence network model was used to analyze the interspecific associations of the community. Pearson correlation analysis was conducted between the dominant phytoplankton species and environmental factors. The results showed that a total of 644 species of phytoplankton were identified, belonging to 8 phyla, 12 classes, 25 orders, 49 families and 152 genera. Among them, there were 22 dominant species, and 90.9% of the dominant species were diatoms, which were absolutely dominant in the community. The most abundant (74.193×10⁴ Cell/L) and the highest occurrence frequency (0.867) were found in Synedra ulna during the wet season, which was the absolutely dominant species. The temporal niche width of dominant species (0.833) was higher than that spatial niche width (0.254). The spatial and temporal niche width of the dominant species was mainly affected by the spatial niche width, and spatial heterogeneity was the main factor affecting the distribution of dominant species. The number of niche overlaps of dominant species was 40.69%, and the niche overlaps of dominant species were mainly in middle and low grade. The heterogeneity of demand for spatial and temporal resources among dominant species was high, and the potential competition between species was weak. The network structure of phytoplankton community in dry season was close, and the community connectivity, community complexity and niche overlap between species were higher than those in wet season. The community co-occurrence network of the two hydrological periods was mainly positive, and the synergistic effect between species was stronger than the competition effect. The water environment factors could drive niche differentiation of dominant species in the community, and NO₃-N and water temperature were the main driving factors affecting niche differentiation of dominant species in the middle reaches of the Yarlung Zangbo River.