Phytoplankton are the primary producers in the water column which could indicate environmental changes. The aquatic ecosystems of plateau reservoirs are fragile due to the large evaporation, short rainy season, long dry season and weak resistance to pollutions. Therefore, the dynamics of phytoplankton communities are more sensitive to environmental changes. In order to reveal the community structure and succession characteristics of phytoplankton in Wujiangdu Reservoir in Yunnan-Guizhou Plateau, we investigated phytoplankton community in Wujiangdu Reservoir from June 2021 to May 2022, and classified the phytoplankton according to its functional groups. Results showed that: 1) A total of 106 species/taxa of phytoplankton were identified in the surface water of Wujiangdu Reservoir, belonging to 8 phyla and 67 genera. Bacillariophyta had the greatest biomass, and Chlorophyta had the most abundant species richness. 2) There were significant seasonal differences in physical and chemical properties and phytoplankton communities in the surface water of Wujiangdu Reservoir, but the spatial heterogeneity, referring to different sites in the same season, was insignificant. 3) The surface water of Wujiangdu Reservoir was mesotrophic to moderately eutrophic in spring, summer and autumn, and was mesotrophic in winter. 4) There were 24 functional groups of phytoplankton communities identified in Wujiangdu Reservoir. Based on dominance, the seasonal dynamic pattern was B/C/L0/P (spring)→B/J/L0/P/S1/T (summer)→B/L0/P/S1/T (autumn)→B and P (winter). The correlations between the dominant functional groups in spring and autumn were significant, while in-significant in summer and winter, indicating their relative independence. Therefore, the seasonal dynamics of phytoplankton community was accompanied with the changes of community stability. 5) Nitrogen and phosphorus nutrients, temperature and dissolved oxygen were the key environmental factors affecting the phytoplankton community in Wujiangdu Reservoir. This study provided the background information of phytoplankton community structure in a typical plateau reservoir of Yungui Plateau, described the characteristics and analyzed the primary factors for the seasonal dynamics.