Regarding the problem of setting seawall entrance during the construction of the seawall in nearshore tidal flat areas，a mathematical model of the nearshore tidal flat area is built as a research tool to simulate the hydrodynamic conditions of the tidal flat area where the seawall is located and the exchange between the water enclosed by the seawall and the open sea.We study the relationship of the size of the seawall entrance with the flow velocity at the entrance，the exchange time，and the exchange rate of water enclosed by the seawall.The calculation results show that the nearshore tidal flat areas are affected by the terrain elevation，and most tidal flat areas are exposed during the ebb phase.For the seawall built in this area，when the size of the entrance is more than 100 m，it can not only ensure that the maximum flow velocity at the entrance can be controlled below 1 m·s but also realize that all water inside the seawall can be discharged to the open sea in a tide cycle，achieving an exchange rate of 100% with the open sea water，which provides a decision-making basis for the later setting of seawall entrance.