In order to understand the structural and succession of the phytoplankton functional groups (FGs) and its the ecological driving mechanism during extreme drought in Xinfengjiang Reservoir, the composition of phytoplankton community and physicochemical factors of water quality were investigated monthly in 2021. The results showed that the phytoplankton communities were classified into 23 functional groups during extreme drought period. Among them, functional groups A/E/F/J/K/L_o/M/MP/NA/P/SN/TB/X1/X2/X3/Y occurred higher frequently, and the dominant functional groups were A/E/F/J/L_o/MP/P/X3/Y. The main temporal changes characteristics of phytoplankton FGs occurred for A/E/MP/P/X3→E/L_o/MP→A/E/L_o→A/E/L_o→A/L_o→A/L_o/Y→A/F/L_o/MP/Y→A/F/L_o/Y→A/F/L_o→A/F/L_o/MP→A/E/F/L_o/MP/Y→A/E/L_o from January to December in 2021. The main dominant FGs of Xinfengjiang Reservoir during extreme drought were A/E/L_o/Y, and they were suitable for the oligotrophic water bodies. Spearman correlation and redundancy analysis showed that water temperature and transparency were the primary environmental factors affecting the structural of most phytoplankton FGs during extreme drought in Xinfengjiang Reservoir. The ecological status index during extreme drought of Xingfengjiang Reservoir was considered as in excellent condition, as shown by its ecological status index value ranged from 4.07 to 4.87.