Abstract: To compare and analyze the seasonal variation of phytoplankton community structure and its correlation with environmental factors within the central Bohai Sea and the Bohai Strait, water samples from 25 stations (118°-122°E, 37°-40°N) were collected from the Bohai Sea in spring, summer and autumn of 2021. The phytoplankton species composition, abundance distribution, dominant species, community structure and environmental factors were comprehensively surveyed. A total of 168 phytoplankton taxa affiliated to 68 genera of 3 phyla were identified, among which diatom and dinoflagellate were the most abundant. The average water column cell abundance of phytoplankton in spring, summer, and autumn was 78.76×10³, 18.83×10³, and 25.02×10³ cells/L, respectively, and showed a “two-peak pattern”, with a main peak in spring and a sub-peak in autumn. The spatial distribution of phytoplankton cell abundance differed seasonally. The high-value area was located in the Bohai Strait in spring, but was at the Yellow River Estuary in autumn, was patchy in summer. The diversity index, evenness index and richness index of the community structure in autumn were all higher than those in spring and summer. The spatial-temporal distribution of phytoplankton community structure in the Bohai Sea was affected by multiple environmental factors. Compared with summer, nutrients had more significant effects on phytoplankton in spring and autumn.