Taking the Fengjiangkou Reservoir as research area, sampling and monitoring of phytoplankton were carried out during the flood, normal and dry seasons to analyze the composition structure characteristics and diversity level. By using the comparative analysis method and the uniform mixing model, the differences in phytoplankton community composition between the Danjiangkou Reservoir and Fengjiangkou Reservoir were explored, and the changes in phytoplankton density in the Fengjiangkou Reservoir after the implementation of the North Hubei Project were predicted. The results showed that there were 17 dominant species of phytoplankton in the Fengjiangkou Reservoir, belonging to Cyanophyta, Chlorophyta, Bacillariophyta and Cryptophyta, with Cyanophyta being the dominant one. The density of phytoplankton showed significant differences in time and space. The average total density of all sampling points in the dry season was as high as 51.55 million cells/L, which was significantly higher than that in other periods. In different periods, the total density of dominant algal phyla in the Fengjiangkou River and Xiaohuangjiapang was significantly higher than that in other sampling points,which were the areas that needed to be focused on for abnormal proliferation of algae in the Fengjiangkou Reservoir. The number of phytoplankton species in the Fengjiangkou Reservoir was significantly higher than that in the Danjiangkou Reservoir, and the diversity level was also better than that in the Danjiangkou Reservoir. In the Danjiangkou Reservoir, the total density of phytoplankton was dominated by Cryptophyta. The implementation of the North Hubei Project could play a certain regulatory role in the density of algae in the Fengjiangkou Reservoir. However,after the implementation of the project, the total nitrogen mass concentration in the Fengjiangkou Reservoir increased by 40% to 130% month by month, and the total phosphorus mass concentration increased by 10% to 30% from August to November. It is suggested that August, September and November be listed as key monitoring periods to avoid abnormal reproduction of dominant phytoplankton species due to increased nutrients,which may lead to the risk of eutrophication.