During construction of port projects, it will produce tremendous economic benefit to use dredged materials for land reclamation. Meanwhile, it will affect the exchange of seawater and change local marine hydrodynamics, thus it will exert an adverse effect on the marine environment. In order to protect the marine resource, we shall grasp the situation of water exchange after reclamation. Based on numerical simulation, and according to the international codes, we analyze the variation law of water. By calculating the width of the flow crosssection,optimizing the design of water culverts to accelerate the exchange of seawater, we achieve the objective of protecting the quality of seawater.