To elucidate the community structure of phytoplankton, the primary producers, and associated environmental drivers in the Rushan oyster aquaculture area, field investigations were conducted at 20 fixed stations during four cruises in Oct. 2023, Feb. 2024, May 2024, and Aug. 2024, which represented autumn, winter, spring, and summer, respectively. Qualitative samples were collected via vertical tows using a Shallow Water Ⅲ plankton net, while quantitative samples were obtained from surface waters (0.5 m) using a 5 L organic glass water sampler. All samples were fixed with Lugol′s solution. Environmental parameters, including water temperature, salinity, pH, and dissolved oxygen, were measured in situ using a YSI professional plus portable multiparameter meter, and secchi depth was determined with a 30 cm Secchi disk. Chlorophyll a was analyzed by acetone extraction-spectrophotometry, and nutrients (NH₄⁺-N, NO₃^--N, NO₂^--N, PO₄^3--P, and SiO₃^2--Si) were determined spectrophotometrically. Community diversity indices were calculated using PRIMER 6.0, and relationships between environmental factors and phytoplankton assemblages were analyzed using DCA and RDA in Canoco 5.0. A total of 67 species belonging to 34 genera and 2 phyla were identified, with an average abundance of 25.39×10³ cells/L. Diatoms (Bacillariophyta) dominated the species composition of phytoplankton (91.04% of the total). Seasonal analysis revealed significant fluctuations in species richness and abundance. The richness peaked in autumn 2023 (37 species) but bottomed out in summer (23 species). The abundance also peaked in autumn (59.50×10³ cells/L), which was significantly higher than that of other seasons (P < 0.01). Diversity indices remained consistently high (Shannon-Wiener H′=2.51±0.48, Margalef d=2.63±0.93, Pielou′s J=0.86±0.07), indicating that high diversity and even distribution were significantly variable seasonally (P < 0.01). Twelve dominant species, all diatoms were identified weirdly (Y≥0.02). Coscinodiscus granii dominated autumn through spring while Pseudo-nitzschia pungens dominated winter through summer. Pearson correlation and RDA revealed that phytoplankton community structure was driven by multiple physicochemical factors which included chlorophyll a, dissolved oxygen, temperature, nutrients (N, P, Si), salinity and COD_Mn, and the dominant driver(s) shifted seasonally. These findings provide a scientific basis for ecological conservation and sustainable management of the Rushan aquaculture area.