In order to study the impacts to phytoplankton caused by fish cage farming activities, three seasonal investigations (winter, spring and autumn) on phytoplankton community and environmental factors in the Hailing Bay were carried out. In total,171 phytoplankton species were detected, with 76 species of diatom belonging to 29 genera, followed by dinoflagellates represented by 16 taxas (8 genera), blue-green algae by 4 taxa (2 genera).The range ofphytoplankton density was 22-3.054×10⁵ cells/L, which ranked as:autumn>spring>winter. In general, the phytoplankton density in the outer bay was higher than that in the inner bay except in spring. Themain dominant species were Thalassiosira subtilis and Thalassiothrix frauenfeldii in winter, T. subtilis and Skeletonema costatum in spring, S. costatum, and T. frauenfeldii in autumn. The range of Shannon-Wiener diversity index (H') were 1.44-4.11, 0.66-2.39 and 1.62-3.10 in winter, spring and autumn. Redundancy analysis suggested that the main variables affecting the dominant species were water salinity and pH in winter, ammonia (NH₄-N) and pH in spring, dissolved oxygenand nitrate (NO₃-N) in autumn.Under the influence of different runoff and aquaculture activity, most of the phytoplankton communities in autumn were easily clustered into similar communities; however, those in spring tended to be more varied. Close relationshipwas found among the results of cluster analysis of stations on phytoplankton community structures and environmentalfactors.