Abstract: Phytoplankton samples were collected from 24 stations in Sansha Bay, Fujian Province in April 2019.The species composition, dominant population, cell density and ecological index of phytoplankton were preliminarily studied. The results showed that 181 species belonging to 69 genera and 4 phyla were identified in Sansha Bay, including 161 species in 57 genera of Bacillariophyta, 16 species in 10 genera in Dinoflagellates, 1 genus and 3 species in Cyanophyta, 1 genus and 1 species in Chrysophyta. The dominant species were Paralia sulcata, Skeletonema costatum, Thalassionema nitzschioides, Coscinodiscus jonesianus and Nitzschia sp. The cell density of phytoplankton in the middle layer was higher than that in the bottom layer, and the density of phytoplankton varied greatly in the horizontal distribution. The density ranged from (9.2~38.9)×106 cells/m3, with an average of 22.67×106 cells/m3.The richness index of different stations was between 0.77~1.83, with an average of 1.24. The evenness index was between 0.35~0.74, with an average of 0.63. The diversity index was between 1.14~2.63, with an average of 2.13. And the overall evaluation was in the middle level.