Based on the HD module and the Ecolab module of MIKE3 numerical model considering water baroclinic,the numerical simulation of hydrodynamic environment in the Yangtze River Estuary and adjacent waters is carried out.The verified model is used to analyze the influence of the North Branch middling narrowing project on the hydrodynamic environment.The results show thatthe flow velocity of the North Branch is reduced to a certain extentafter the narrowing project,and the flood velocity nearthe branch pointhas a tendency to increasein the upper segment of the North Branch,which is affected by the tides in the South Branch.The North Branch middling narrowing makes the salinity of the North Branch decrease obviously,and plays a controlling role on saltwater intrusion from the North Branch into the South Branch.The content of phosphate,inorganic nitrogen and chlorophyll-a obviously increases in the lower segment of the North Branch after the implementation of the project.The average concentration of phosphate and inorganic nitrogen decreases slightly,while the content of chlorophyll-a increases in the middle and upper segment of the North Branch.The North Branch middling narrowing has little effect on the hydrodynamic characteristics and nutrient content in the South Branch and offshore area.