Abstract: Based on the five voyages observed data measured from July 2020 to March 2021 of the suspended aquaculture farm off Gouqi island in Zhejiang province, the characteristics of phytoplankton community inside and outside the farm and the relationship between the dominant species and variations of environmental factors were analyzed. Totally 146 species of phytoplankton belonging to 63 genera and 4 phyla were identified in the research sea area. The dominant species mainly include Skeletonema costatum, Noctiluca scintillans, Paralia sulcata etc. The dominant species composition, index of Shannon-Wiener, Pielou and Margalef changed significantly with seasonal variation. The cell abundance of phytoplankton cells showed a trend of July>October>September>December>March. In terms of spatial distribution, in the season of high phytoplankton biomass, the average cell abundance of phytoplankton in the water layer inside the farm where shellfish are cultured at a depth less than 5 m is significantly lower than that in the water layer outside the farm, and also significantly lower than that in the whole water layer inside the farm, which can be both more than one order of magnitude lower at most. The above phenomenon is particularly obvious in July and October when phytoplankton biomass is high. In winter with low biomass, cooling convection leads to strong vertical mixing of water column, and mussels reduce the filtration rate due to low water temperature and increased turbidity. Therefore, there is little difference in phytoplankton abundance between inside and outside the farm and between the surface and bottom. Cultured mussel change the composition of phytoplankton population and the circulation rate of water nutrient by filtration and excretion respectively, which directly or indirectly affects the spatiotemporal distribution of phytoplankton community in the farm and makes the phytoplankton community structure inside the farm more stable than that outside the farm due to top-down or bottom-up control.