The effective width of breakwater entrance and its dimensions are clearly defined in the current design code. However, with the enlargement of ships, some existing breakwater entrances can hardly meet the code-specified navigation requirements. Furthermore, the practice shows that, under some certain sailing conditions, the appropriately reduced effective width of breakwater entrance is able to meet the requirements of safe navigation. On the basis of a project instance, this paper studies relevant specifications and main impact factors of effective width of breakwater entrance, and gives some proposals on methods for determining the effective width of breakwater entrance.