Abstract: Polybrominated diphenyl ethers (PBDEs) pose significant threats to the growth of marine organisms and even human health. However, the migration, transformation, and regulatory mechanisms of PBDEs remain unclear in China’s coastal waters. This study utilizes a numerical model to quantitatively analyze the seasonal migration and transformation of polybrominated diphenyl ethers 153 (BDE-153) in Jiaozhou bay. The simulation results revealed that BDE-153 concentration exhibited apparent seasonal variation with a peak in summer, with the highest concentrations for dissolved and total particulate BDE-153 reaching 0.22 μg/m³ and 2.3 μg/m³, respectively. Transformation rates of BDE-153 during phytoplankton absorption, mortality, and debris mineralization were also significantly higher in summer than in other seasons. Both concentration of BDE-153 and its transformation rates during biotic processes showed a belt-shaped decrease from north to south with the highest values near the DaGu river in the northwest and the MoShui river in the northeast. Rivers were the key source of BDE-153, with an input flux of 10.5 kg/a, significantly higher than the supply rate from air-sea exchange. Convective exchange with the open sea was the primary sink with output rates of 1.1 kg/a and 8.0 kg/a for dissolved and total suspended particulate forms, respectively.