Abstract: In order to investigate the seasonal changes and key driving factors of phytoplankton community structure and functional groups in the Songhua Lake, the phytoplankton and water environmental factors at 12 sampling points were investigated in summer and autumn of 2020, and spring and summer of 2021. Functional group classification, comprehensive nutrition state index, cluster analysis, non-metric multidimensional scaling analysis (NMDS) and redundancy analysis (RDA) were employed to analyze the succession characteristics of phytoplankton community structure. The results showed that species of phytoplankton were identified in the Songhua Lake from 2020 to 2021, 108 species of phytoplankton belonging to 8 phyla were detected, among which Chlorophyta accounted for the highest proportion (41.67%), followed by Bacillariophyta (26.85%). Thirteen dominant species were found during the investigation, and Aphanizomenon sp. was the only common dominant species in the four investigations. Phytoplankton in the Songhua Lake were classified into 23 functional groups, with 5 dominant functional groups including P, Lo, Y, MP and D, which shared the common feature of adaptation to medium to high eutrophic water. In terms of time series, changes of dominant functional groups from 2020 to 2021 were P/M/Y/D/B/H1-Y/B/D/X2/P-MP/Y/Lo/P/D-Lo/P/H1/Y/MP, P and Y were the common dominant functional groups in each region of reserve. The results of RDA analysis showed that the eigenvalues of the first and second ordination axes were 17.34% and 11.90%, respectively. Water temperature, pH and total nitrogen were the main factors driving the distribution pattern of phytoplankton functional groups. The results of this study provided data for the management of aquatic ecology in the Songhua Lake.