In order to investigate the community characteristics of phytoplankton and the determining factors in Lake Dongping after implementation of the east route of South-to-North Water Diversion Project, seasonal samples of phytoplankton and aquatic environment from 18 sampling sites were collected in 2017. A total of 191 taxa of phytoplankton (genera or species) were identified with 64 taxa of Bacillariophyta, 57 taxa of Chlorophyta, 28 taxa of Euglenophyta, 22 taxa of Cyanophyta, 8 taxa of Chrysophyta, 5 taxa of Xanthophyta, 4 taxa of Pyrrophyta and 3 taxa of Cryptophyta. The dominant taxa were Limnothrix planctonica and Chroococcus sp. of Cyanophyta, Chlorella vulgaris of Chlorophyta and Cyclotella sp., Nauicula sp. and Synedra sp. of Bacillariophyta, etc (Y>0.02). The phytoplankton density and biomass were 6.74×10⁶ cells/L and 5.71 mg/L with the seasonal variation of density being summer > autumn > winter > spring and that of biomass being summer > winter > autumn > spring, respectively. About the phytoplankton diversity in four quarters, the number of species was 71, 78, 53 and 125, while the averages of the Shannon-Wiener index were 1.90, 1.84, 1.76 and 2.52 and Margalef diversity index were 3.07, 3.98, 2.79 and 4.75, respectively. Meanwhile, the results of redundancy analysis and Pearson analysis for the relationship between phytoplankton and aquatic environment showed that the factors such as comprehensive nutrition state index, permanganate index, ammonia nitrogen concentration, and water temperature played a key role in affecting the structure and dynamics of the phytoplankton community in Lake Dongping. Compared with before implementation of the east route of South-to-North Water Diversion Project and the initial stage of it, although the density of phytoplankton showed a rise, there was no obvious response in their dominant genera. This study preliminarily verified the spatiotemporal distribution characteristics of the phytoplankton community in Lake Dongping. Total phytoplankton density was found to have increased with the change of water quality caused by the east route of South-to-North Water Diversion Project. The withdrawal of aquaculture and implementation of fishery enhancement and releasing had reduced the risk of eutrophication, but the proliferation of phytoplankton probably increased the risk of algal-bloom in Lake Dongping. Given the changes of phytoplankton community and water quality after water diversion, it is necessary to strengthen the ecological monitoring and controlling in the management of water quality safety and stability.