Abstract: To investigate the phytoplankton community structure and assess the aquatic ecological health of Andi Reservoir in Zhejiang Province, four seasonal samplings were conducted from September 2022 to July 2023. The results showed that a total of 62species belonging to seven phytoplankton phyla were identified. The total cell density varied from 1.30×10⁵ to 2.95×10⁶ cells/L, with a mean value of 9.37×10⁵ cells/L. The biomass varied from 0.14 to 1.36 mg/L, with a mean value of 0.52 mg/L. The dominant species were Microcystis sp. and Chroococcus sp. Redundant analyses indicated that phytoplankton biomass was primarily affected by water temperature, water depth, transparency, and permanganate index. Phytoplankton were classified into 21 functional groups, of which functional group L_O was dominant in all seasons except autumn. The ecological condition assessment based on the Q index of functional groups indicated that the reservoir was generally in good condition, with the lowest in January (good) and the highest in September (excellent), respectively. According to trophic level index (TLI), the reservoir was in mesotrophic condition, regardless at seasonal and spatial scales. Compared with TLI, Q index can better distinguish seasonal changes in ecological conditions in Andi Reservoir, and better reflects changes in its nutrients concentrations. In this study, we characterized the phytoplankton community structure in Andi Reservoir and assessed its ecological health using Q index and TLI. The findings provide a certain reference for the ecological management and health assessment of the reservoir.