Abstract: The Yanghe Reservoir is an important drinking water source for Qinhuangdao City. In recent years, severe blooms of cyanobacteria in this reservoir have become a growing threat to the water safety. To study the temporal and spatial patterns of the phytoplankton growth in Yanghe Reservoir, we continuously collected water samples from six sites throughout one year with a higher sampling frequency during the summer. We identified 49 phytoplankton species that belonged to 41 genera and 8 phyla. The seasonal variation of community structure was closely related to the temperature. Cryptomonas erosa of Cryptophyta was the dominant species in the spring. Oocystis borgei of Chlorophyta was the dominant species in the early summer when the temperature of surface water was below 25℃. Microcystis spp. of Cyanophyta later became dominant when the water temperature rose above 25℃. Fragiaria crotomensis of Bacillariophyta and C. erosa were the dominant species in the autumn. The total biomass of both Cryptophyta and Diatom in the north part of the reservoir was higher than that in the south part. Due to the impact of southeast wind, the biomass of cyanobacteria in the west river mouth (S2) was the highest. CCA analysis showed positive correlation between the biomass of Microcystis spp. and the nitrogen level. Anabaena spiroides was only found in west river mouth and was short-lived. The appearance of Anabaena spiroides could depend on the phosphorus level.