Abstract: The Beiyun River basin was selected as the study area, based on sampling data collected in 2024, this study systematically investigated the phytoplankton community structure, ecological niche characteristics of dominant species, niche overlap, interspecific associations, and their relationships with water environmental factors. The results showed that: (1) A total of 156 phytoplankton species belonging to 7 phyla were identified, and 14 dominant species were selected, including 6species from Cyanophyta, 4 from Chlorophyta, 2 from Bacillariophyta, and 1 each from Euglenophyta and Cryptophyta. The number of dominant species was relatively high in spring and summer but decreased in autumn and winter. Cyclotella meneghiniana was identified as a dominant species throughout the year and exhibited absolute dominance in winter. (2) Niche breadth ranged from 0.074 to 0.464, while niche overlap ranged from 0.021 to 0.986. The proportion of species pairs exhibiting high niche overlap exceeded 66% in spring, autumn, and winter. Niche characteristics differed among species and seasons. Overall interspecific associations among dominant species were significantly positive in spring, summer, and autumn but significantly negative in winter. Positively associated species pairs (24 pairs) substantially outnumbered negatively associated pairs (six pairs). Both positive and negative associations were observed among dominant species, indicating seasonal differences in their co-occurrence patterns. (3) Pearson correlation and RDA analysis indicated that the associations of water temperature, pH, and nutrient concentrations with dominant species distributions and community diversity varied among seasons. These findings highlight seasonal differences in dominant phytoplankton composition and its associations with environmental factors in the Beiyun River Basin. Summer management should prioritize monitoring dominant cyanobacteria and controlling nutrient inputs, while long-term integrated monitoring is needed to support aquatic ecological restoration and eutrophication management.