Abstract: To discern the community structure of phytoplankton in the water of Honghu Lake Basin, this study deployed 10 sampling sites in Honghu Lake in October 2021, and evaluated the phytoplankton community using the diversity indices. The results show that there were 66 species of phytoplankton belonging to 49 genera in six phyla, of which the cyanophyta was the most species, followed by bacillariophyta. The phytoplankton biomass of the Honghu Lake Basin in winter ranged from 7.85~42.45 mg/L, averaging 22.35 mg/L. A comprehensive evaluation of the nutrient status of the lake water was conducted based on Margalef's richness index (M), Shannon-Weiner's diversity index (H), Simpson's diversity index (D), Pielou's evenness index (J), and the species dominance index (Y), suggesting that the Honghu Lake Basin water was moderately to severely polluted. This study holds that the water quality of the Honghu Lake Basin can be improved from the following aspects: ① The water protection of the Honghu Lake Basin should be enhanced by strictly controlling the inflow of various water pollutants from downstream and surrounding areas, implementing timely and efficient prevention and control of rural water pollution in the lake area, and properly organizing the balanced harvesting of aquatic phytoplankton; ② The water environment treatment should be strengthened to gradually restore the self-purification capacity of the lake water; ③ The publicity efforts should be intensified by regularly conducting educational activities on water pollution prevention and control in rural areas, residential communities, and schools.