Abstract: In view of the actual project of the composite ecological purification system in an estuary of Yangcheng Lake, based on exploring the seasonal composition characteristics of the phytoplankton community in each purification unit, comprehensive evaluation and analysis of water quality health status were carried out by using comprehensive trophic level index (TLI) and phytoplankton diversity index. The results showed that a total of 143 species of phytoplankton from 97 genera and 8 phyla were identified in the purification system, with Diatoma, Chlorophyta, and Cyanophyta dominating the species composition. The phytoplankton intensities of the influent and effluent of the system were 5.81×10⁵-1.76×10⁷ cells/L and 4.96×10⁵-1.65×10⁷ cells/L, respectively, and the biomass ranges were 0.292-5.21 mg/L and 0.194-4.66 mg/L, respectively. The system had an effective control over phytoplankton growth in the water body. According to FG functional group classification method, the phytoplankton in the purification system could be divided into 26 functional groups, among which B, D, MP, P, S1, W1, X2, Y, G, J, L_O, and M were the dominant functional groups. The succession of dominant functional groups in each unit correlated well with water quality indices such as water temperature, COD_Mn, DO, and TN. The water body of the composite purification system was in mesotrophic to slightly eutrophic state during the research period, indicating that the system effectively improved the ecological health of the water entering the lake.