Abstract: According to the seasonal investigations of phytoplankton in May(spring) and August(summer) of 2020,the community structure and biodiversity of important mariculture area of Dongshan Bay were studied.A total of 74 phytoplankton species in 40 genera and 3 phyla was identified.Bacillariophyta dominated the phytoplankton community,followed by Pyrrophyta and Chrysophyta.The species compositions were changed little both in seasonal and spatial distribution.Phytoplankton density ranged from 1.02×10⁵ to 3.18×10⁵ cell/dm³ in different survey stations.The phytoplankton's densities were changed little with seasons though they varied spatially with sties.The dominant species were Skeletonema costatum, Thalassionema nitzschioides and Pleurosigma acutum.The diversity indexes of Margalf,Shannon-Weaver and Pielou were 1.56~2.61,2.38~3.48 and 0.55~0.91,respectively.The assessment of phytoplankton's diversity indices and eutrophication showed that the water quality of important mariculture area of Dongshan Bay was not severely polluted,the eutrophication was at medium nutrient level,but the risk of red tide couldn't be excluded.