Abstract: Phytoplankton are one of the major components of lake aquatic ecosystems. because they are very sensitive to changes in water environments, they are often used as indicator species to monitor water conditions. However, how these environmental changes are responded to are dependent on the classification approach chosen. The study site for this study is Lake Maoli, and 12 sampling sites (S1-S12) were set up throughout the entire lake. Phytoplankton samples were collected with the zooplankton sample in 2020 on April 27 (spring), July 30 (summer), and October 28 (autumn), and on January 25, 2021 (winter). Taxonomic groups (phylum, genus) and functional groups two kinds of classification methods were used to analyze the community structure of phytoplankton, and biodiversity indexes and the phytoplankton community index (Q index) were used to evaluate the water quality of Maoli Lake. From the results of the study, the phytoplankton in Lake Maoli included 7 phyla and 57 genera, with Chlorophyta being dominant (27 species, 47.37%) and Cyanophyta (12 species, 21.05%). The most common taxa covered five phyla and 29 genera, and can be grouped into 24 groups and 7 dominant groups. The succession of species in different seasons was “diatom-green algae” type (spring) - “cyanobacteria-green algae” type (summer) - “cyanobacteria-green algae” type (autumn) - “diatom-euglenophyte” type (winter); the succession of functional groups in different seasons was D/L0/W1/P (spring) -G/MP (summer) -G/W1 (autumn) -W1 (winter). Use Shannon-Wiener diversity index, Pielou evenness index, and Margalef richness index to assess water quality of phytoplankton in Lake Maoli, with the average D index being 1.88, suggesting an α-mesosaprobic (moderately polluted) state; the average J index being 0.71, suggesting an oligosaprobic or unpolluted state; and the average H’ index being 2.19, suggesting a β-mesosaprobic (moderately polluted) state. In summary, it can be seen that the water body of Lake Maoli is in a oligosaprobic – β-mesosaprobic state. The phytoplankton Q index in Lake Maoli exhibited a trend of first decreasing and then increasing with seasons, reaching the minimum value in autumn; Q index assessment indicated that the trophic level of the water body was moderate-good, indicating a state of moderate eutrophication. Comprehensive analysis showed that the water quality of Lake Maoli is in a mildly polluted-good condition, with mild eutrophication, and both the phytoplankton functional group Q index and the traditional biodiversity index method have relatively good application conditions for water quality evaluation of Lake Maoli, but the functional group classification method is more sensitive, especially in terms of reflecting water eutrophication. This study systematically uncovered the succession patterns of phytoplankton functional groups in Lake Maoli and the primary drivers (such as water temperature, water nutrient levels, and the feeding activity of silver and bighead carp) associated with these groups, and the obtained results supply key data and a scientific foundation for targeted management of aquatic ecosystems, eutrophication prevention and control, and ecological protection strategy formulation.