In the use of a tidal mathematical model to optimize the layout of the encircled breakwater in Xuwei port, the maximal velocity in the port area is considered as a very important index. It is found through analysis that the maximum velocity is related to the waterside area, the flow section area of port entrance, and the tidal range and last of flood tide. According to the simulation results of the mathematical model, we develop an empirical formula to calculate the maximum velocity, which can be used to estimate the design plan in preliminary evaluation.