Abstract: To investigate the dynamic relationship between phytoplankton communities and physicochemical indicators in Xinfang Reservoir of Yunnan Province, monthly sampling was conducted from January 2021 to December 2023. Phytoplankton community structure was identified by microscopy, combined with physicochemical parameter analysis. Shannon-Wiener diversity index, Pielou evenness index and Pearson correlation were used to evaluate ecological status and influencing factors. The results indicated that: ① A total of 6 phyla and 33 genera of phytoplankton were identified, with Cyanophyta (27.27%), Chlorophyta (42.42%), and Bacillariophyta (15.15%)as dominant groups.② The annual average cell density was 40.70×10⁴ cells/L, showing seasonal variation of autumn>summer>winter>spring.③ Diversity index (H=2.21)and evenness index (J=0.61), the phytoplankton community in Xinfang Reservoir maintains high overall stability. However, cyanobacteria exhibit a long-term dominance in cell density compared to other algal groups, and their seasonal blooms could trigger eutrophication risks.④ Phytoplankton density was significantly positively correlated with COD_Mn and chlorophyll-a (p < 0.05), and highly correlated with pH and sulfate (p < 0.01). The findings of this study can provide a reference for ecological health assessment and management of the reservoir.