Abstract: In order to understand the characteristics of phytoplankton community structure and water quality conditions in Liushui Bay of Pingtan in spring and summer, the species composition, cell density and biological indicators of the phytoplankton community in Liushui Bay of Pingtan were investigated in March (spring) and June (summer) of 2021. The results showed that there were 71 species of 35 genera in 3 phyla in Liushui Bay, among which the species number of Bacillariophyta was the highest, with 49 species of 28 genera. The number of species changed significantly in time and space, and the number of species in summer was greater than that in spring. The cell abundance ranged from 3.26×10⁵ cells·m^-3 to 10.09×10⁷ cells·m^-3, which was greater in summer than that in spring. The main dominant species of phytoplankton were Coscinodiscus jonesianus,Nitzschia sp. and Karenia mikimotoi. The species diversity index of different stations ranged from 0.78 to 3.56, the species richness index ranged from 0.818 to 3.92, and the evenness index ranged from 0.17 to 0.85. According to the phytoplankton diversity index, the overall evaluation of water quality in Liushui Bay in spring and summer was mild pollution to moderate pollution. According to the nutritional evaluation of water quality, the water in spring and summer was oligotrophic. In spring and summer, the toxic red tide organism Karenia mikimotoi appeared in LiuShui Bay of Pingtan, which had the potential risk of fishery loss.