Abstract: This study utilized pigment-based classification to analyze the spatial distribution characteristics of phytoplankton community structure in the central and adjacent areas of the Bohai Sea in July 2021, and analyzed the effects of environmental factors. The results showed that there were significant spatial differences among phytoplankton assemblages in the Bohai Sea in summer. The dominant phytoplankton assemblages in the surface layer were Synechococcus and Chlorophyceae, accounting for (30.36 ±18.17)% and (23.60±6.60)% of total Chlorophyll a (Chl a) concentration, respectively. The dominant phytoplankton assemblages in the middle layer were Bacillariophyceae and Chlorophyceae, accounting for (24.43±9.76)% and (21.04±5.56)% of total Chl a concentration, respectively. The dominant phytoplankton assemblages in the bottom layer were Bacillariophyceae and Prasinophyceae, accounting for (35.28±8.49)% and (19.16±6.04)% of total Chl a concentration, respectively. The high value area of Synechococcus was mainly located in the surface layer of Laizhou Bay and Liaodong Bay, while the high value area of Chlorophyceae was located in the surface and middle layer of Bohai Bay and Liaodong Bay. The high biomass of Bacillariophyceae was mainly located in the middle and bottom layer of Bohai Bay and Laizhou Bay, while Prasinophyceae was mainly distributed in the bottom layer of Bohai Bay and Liaodong Bay. During the survey, obvious stratification phenomenon existed in the central Bohai Sea, and most areas were limited by phosphorus (P), but not by nitrogen (N) or silicon (Si). Correlation analysis with environmental factors showed that relatively low sea temperature (T) and nitrate were the most important factor affecting the biomass of Bacillariophyceae, while the relatively high temperature and phosphorus limitation in surface layer may be the reason for the dominance of Synechococcus and Chlorophyceae.