In order to study the structure and function of phytoplankton community and its ecological significance in eutrophic lake- Chaohu Lake,20 sampling sites were set up in the main estuary and pelagic area of Chaohu Lake. The survey was conducted for four quarters between August 2020 and April 2021. The distribution and influencing factors of phytoplankton functional groups in the estuary and pelagic area of Chaohu Lake were also analyzed. A total of 90 species of phytoplankton belonging to 42 genera and 8 phyla were identified in the estuary area and 80 species belonging to 42 genera and 8 phyla were identified in the pelagic area. The average abundance, biomass, Margalef richness index and Shannon-Wiener diversity index in the estuary area were higher than those in the pelagic area. There were significant differences in abundance (P＜0.05), but no significant differences in biomass, Margalef richness index and Shannon-Wiener diversity index were significantly different (P＜0.01). According to FG functional group classification method, the composition of phytoplankton functional groups in the estuary area was similar to that in the pelagic area. According to the calculation of dominance degree, there were 5 dominant functional groups M, Lo,