Abstract：The phytoplankton community in Jinze Reservoir was investigated from July 2018 to June 2019 to study the temporal and spatial changes of the phytoplankton FG functional group and their relationship with environmental factors and explore the reasons for the similarities and differences in the characteristics of phytoplankton communities at the inlet and outlet. During the investigation period， the water body was at mild to moderate eutrophication. A total of 139 phytoplankton from 8 phyla were detected in the reservoir， mainly composed of species of Cyanophyta， Chlorophyta and Bacillariophyta， and a total of 25 functional groups. The dominant functional groups in the reservoir were M， H1， LO， and K. All the four groups prefer eutrophic water environments， and the main phylum is Cyanophyta. The M functional group， mainly constituted by Microcystis， is the dominant functional group. There were 7 phyla and 87 species in the inlet water， 7 phyla and 102 species in the outlet water. These species belong to 23 groups and 20 functional groups. There are five groups and nine groups of dominant function groups in the inlet and outlet waters， respectively. The species similarity of the inlet and outlet waters is higher than 0.6， which is moderately similar. The dominant function groups are M， H1， LO， K in the inlet and outlet waters， mainly cyanobacteria. Functional group M is the dominant functional group in the inlet and outlet waters. The monthly biological density variation in the reservoir ranges from 280.1162×104 to 1750.7050×104 cells·L-1. The annual average biological density in the outlet water is 29.78% lower than that of the inlet water， but the difference is not significant. The biological density of Cyanophyta determines the monthly variation of the algal biological density in the reservoir and the difference of the annual average value of the algal biological density at the inlet and outlet. The results of redundancy analysis show that the environmental factors explained 53.59% variances of the phytoplankton in the reservoir. The M functional group， which had the absolute predominance， was greatly affected by and positively correlated with total nitrogen.Results show that a series of purification measures in the reservoir have a certain effect， but due to the influence of incoming water， the reservoir still has the risk of algal blooms. It is necessary to adjust the water quality preservation strategy in the reservoir and strengthen the purification of water quality to ensure the safety of drinking water.