The features of coastal upwelling in the southwestern Yellow Sea were investigated based on oceanology data from a research cruise and a regional circulation model. The observation data suggest that a relatively colder and saltier water core exists from the deeper layer to the surface, off the Subei Bank. The concentrations of nutrients also suggest that coastal upwelling is beneficial for nutrient enrichment in the upper layer. The numerical simulations are in good agreement with oceanology observations. Furthermore, sensitivity experiments indicate that, in addition to the tidal-induced upwelling and tidal mixing proposed in previous studies, the summer monsoon is also critical to vertical circulation in the southwestern Yellow Sea. The southwesterly wind stress and positive wind stress curl make considerable contributions to upwelling off the Subei coast compared with tidal motions. Moreover, this study also proposes that changes in the summer monsoon and its curl may have been helpful to the formation of upwelling during the past decade, which may have provided a favorable marine environment for the frequent occurrence of green tides. This study provides a theoretical basis for the mechanisms of coastal upwelling and the nitrogen cycle in the Yellow Sea.