Abstract: Phytoplankton and environmental factors are important components of water ecology, but there are currently few studies on the relationship between phytoplankton and environmental impact factors in plateau rivers. Therefore, Tanglang River-Pudu River basin in the Yunnan-Guizhou Plateau was taken as the research object, and the phytoplankton and water bodies in the river basin were sampled and investigated from April to November (dry season, wet season, and level season) in 2022. The diversity index, comprehensive trophic state index, and redundancy analysis (RDA) were used to analyze the water quality of Tanglang River-Pudu River basin and the relationship between phytoplankton and environmental factors, and to explore the characteristics of phytoplankton communities in the basin and their relationship with environmental factors. The results showed that a total of 261 species belonging to 128 genera and 8 phyla of phytoplankton were detected, mainly including Cyanophyta, Chlorophyta and Bacillariophyta. Aphanizomenon flos-aquae was the dominant species throughout the year. The spatial and temporal changes of phytoplankton community structure and diversity were significant. Compared with the dry season and the level season, the cell density in the wet season was less and the biodiversity was higher. The cell density in the mainstream of the Tanglang River was higher than that in other basins. The comprehensive nutritional status index indicated that the whole basin was at the level of medium nutrition to heavy eutrophication, and the eutrophication level was the severest in the mainstream of the Tanglang River . The RDA analysis revealed that different sub-watersheds had different dominant factors influencing the community structure of phytoplankton. For the mainstream of the Tanglang River, the dominant factors were permanganate index (COD_Mn) and pH. For the tributary of the Tanglang River, the dominant factor was COD_Mn. For the mainstream of the Pudu River, the dominant factors were flow (Q) and nitrate nitrogen (\mathrmNO_3^- -N). For the tributary of the Pudu River, the dominant factor was water temperature (WT).