Abstract: From May 2022 to February 2023, four quarterly surveys were conducted to investigate the dynamic characteristics of phytoplankton functional groups and their relationships with environmental factors in the emergency water supply for East Taihu Lake. The results showed that: (1) A total of 128species belonging to 7 phyla of phytoplankton were identified and classified into 21 functional groups, including MP, P, and S1, which prefer high nutrient disturbance, as well as H1 and W1, which prefer high nutrient and low disturbance environments. The L_O group exhibited wide adaptability. Functional group species was the higher in summer and autumn, with autumn showing significantly higher biomass, suggesting the water habitat was shallow, weakly stratified, less transparent, and in a high trophic state. (2) Seven dominant functional groups (E, H1, L_O, MP, P, S1, and W1) were identified, displaying clear seasonality and a succession pattern: spring (L_O/W1/P)→summer (H1/S1/MP)→autumn (L_O/P/MP)→winter (E/H1/L_O). The dinoflagellates-dominated L_O group and the diatom-dominated MP group constituted the largest proportion of the total. (3) RDA and VPA analysis showed that WT, \rmNH^+_4 -N, TP, and Cond were the main environmental factors affecting the biomass of dominant phytoplankton groups. Functional groups L_O was negatively correlated with WT, \rmNH^+_4 -N, TP, and Cond, while groups W1, S1, and MP showed positive correlations with these variables. (4) The Q-index evaluation of functional groups (2.91±0.11) indicated that the overall water quality of the source was “medium”. However, during the summer, water quality deteriorated, with high nutrient levels and elevated cyanobacterial abundance, indicating an ongoing risk of cyanobacterial blooms. It is recommended to strictly control external nutrient input, strengthen ecosystem regulation, and enhance the stability and self-purification ability of the ecosystem to mitigate bloom occurrences and ensure water supply safety.