Abstract: We studied the phytoplankton community structure and the environmental factors in different hydrological periods during the wet season in 2020 and the wet and dry season in 2021 of Dongzhang Reservoir including the inlet, the center and the outlet, which locates in Fuqing City of Fujian Province. The results showed that the average total phytoplankton density varied from 2.3×10⁷ to 5.2 ×10⁷ cell/L at the outlet of Dongzhang Reservoir during the wet season in 2021, which was more than three times that of the average phytoplankton density varied from 5.6×10⁶ to 2.3×10⁷cell/L during the dry season. The Bacillariophyta, Cyanophyta and Chlorophyta were the dominated and the average proportions were 47.5%, 31.6% and 14.0%, respectively, in the dry season. The Cyanophyta occupied an absolute advantage with an average proportion of 77.7% in the wet season. The content of the soluble reactive phosphorus (SRP) was low in the water during wet and dry season, but the content of total phosphorus (TP), total dissloved phosphorus (TDP) and dissolved organic phosphorus (DOP) were relatively high. Pearson correlation analysis showed that Cyanophyta, Chlorophyta and total algae density were extremely significantly negatively associated with SRP, and Chlorophyta and Bacillariophyta were significantly positively associated with DOP and TDP. The RDA analysis of environmental factors and phytoplankton density revealed that the Cyanophyta was strongly and positively correlated with the air temperature, and the Bacillariophyta positively correlated with the water level. This study provided a reference for the prevention and control of Cyanobacteria bloom and the protection of drinking water sources in Dongzhang Reservoir.>