Due to limited in situ data and diagnostic numerical models, the summer circulation structure and formation mechanism in the Beibu Gulf have always been in controversy in the past 50 years. Therefore, a new three-dimensional hindcast model was built within the northwestern South China Sea(SCS), forced with the daily averaged wind, heat flux, lateral flux, as well as tidal harmonic and eight major rivers discharges. And the east boundary was set up far away off the Qiongzhou Strait(QS). Lastly, the model results were consistent with not only the synchronous observation data from the project 908 but also the historical observed data. As a result, the summer circulation structure was revealed that the southern Gulf was occupied by an anticyclonic eddy whereas the northern Gulf was dominated by a cyclonic gyre. Although the circulation major structure was stable, its area and strength had yearly and monthly oscillation. The other three sensitive experiments indicated that the circulations in the southern and northern Gulf were driven by the SCS circulation and monsoon wind, respectively. After the theoretical analysis of the potential vorticity budget, it was further revealed the circulation in the northern Gulf was driven by the positive wind stress curl in summer. Besides, the river discharge was also significant as the vertical circulation had two layer structures outside the mouth of the Red River. Generally, this work calls for the further research on other subjects, such as ocean biogeochemical or marine fisheries. Due to limited in situ data and diagnostic numerical models, the summer circulation and formation mechanism in the Beibu Gulf have always been in controversy in the past 50 years. Therefore, a new three-dimensional hindcast model was built within the northwestern South China Sea (SCS), forced with the daily averaged wind, heat flux, lateral flux, as well as tidal harmonic and eight major rivers discharges. And the east boundary was set up far away from the Qiongzhou Strait (QS). Lastly, the model results were consistent with not only the synchronous observation data from the project 908 but also the historical observed data. As a result, the summer circulation was was found that the southern Gulf was occupied by an anticyclonic eddy but the northern Gulf was dominated by a cyclonic gyre Although the circulation major structure was stable, its area and strength had yearly and monthly oscillation. The other three sensitive experiments indicated that the circulations in the southern and northern Gulf were driven by the SCS circulation, respectively. respectively. After the theoretical analysis of the potential vorticity budget, it was further revealed the same circulation the northern Gulf was driven by the positive wind stress curl in summer. Besides , the river discharge was also significant as the vertical circulation had two layer structures outside the mouth of the Red River. Generally, this work calls for the further research on other subjects, such as ocean biogeochemical or marine fisheries.