In order to explore the temporal and spatial variation characteristics of phytoplankton functional groups in Qingcaosha Reservoir, Shanghai, and their relationship with environmental factors, as well as the similarities and differences between the characteristics of phytoplankton communities at the inlet and outlet and the reservoir, monthly phytoplankton surveys were conducted in 2021, and the data were analyzed using the phytoplankton functional group （FG） method and redundancy analysis （RDA）. The results showed 143 species of phytoplankton were identified in Shanghai Qingcaosha Reservoir during the investigation. The average biomass of phytoplankton was （0.64±0.35） mg/L. There were 25 functional groups of phytoplankton in the reservoir, among which K, Lo, H1, S1, B, D, P, MP, F and G were dominant functional groups. There were 3 phyla and 40 species in the inlet water, 5 phyla and 48 species in the outlet water. These species belonged to 15 groups and 19 functional groups. There were five groups and nine groups of dominant function groups in the inlet and outlet water, respectively. The average annual biological density and biomass of the inlet water were 118.17×10⁴cell/L and 0.16 mg/L, respectively. Compared with the inlet water, the average annual biological density and biomass of the outlet water increased by 34.85% and 50.12%, respectively, and there was a significant difference between the inlet water and outlet water biomass.The ANOVA analysis showed that there was no significant difference in the composition of phytoplankton functional groups at the head, middle and tail of the reservoir,and the species similarity of the three reservoirs was moderate.Redundancy analysis （RDA） showed that water temperature（WT）, ammonia nitrogen（ NH₄^＋-N） and dissolved oxygen （DO） were the main environmental factors affecting the dominant functional groups of phytoplankton in Qingcaosha Reservoir.In this study, the division and succession of phytoplankton functional groups in Qingcaosha Reservoir were analyzed, and the differences of phytoplankton functional groups in the inlet water and outlet water were compared, which provided a theoretical basis for the protection of water resources in the reservoir.