This study focuses on dynamic mechanism behind the formation of the freshwater zone around the Meimao Sandbank by use of 3D numerical simulation.The Meimao Sandbank is located along the southern bank of the South Passage in the Changjiang(Yangtze River) estuary,which is considered as a freshwater resource for Shanghai City.Interaction between runoff and tide is the main mechanism of the freshwater zone formation.However,the freshwater zone often suffers from saltwater intrusion in dry season.Tidal oscillation is stronger during spring tides,able to carry freshwater farther seaward.Therefore,it is more likely to occur during the ebb of a spring tide in dry seasons.In addition,the water zone is sensitive to runoff:when runoff decreases,it disappears,and vice versa.The northerly winds favor the formation of the freshwater zone. This study focuses on dynamic mechanism behind the formation of the freshwater zone around the Meimao Sandbank by use of 3D numerical simulation. The Meimao Sandbank is located along the southern bank of the South Passage in the Changjiang (Yangtze River) estuary, which is considered as a freshwater resource for Shanghai City. Interaction between runoff and tide is the main mechanism of the freshwater zone formation. Host, the freshwater zone often suffers from saltwater intrusion in dry season. Tidal oscillation is stronger during spring tides, able to carry freshwater farther seaward It is more likely to occur during the ebb of a spring tide in dry seasons.In addition, the water zone is sensitive to runoff: when runoff decreases, it disappears, and vice versa.The northerly winds favor the formation of the freshwater zone.