Taking the expansion project of a port in East Africa as an example，we elaborate the design method of water area dimensions in the narrow harbor，including basin width with the navigational channel function，turning basin diameter，and entrance width.We conduct a comparative study on domestic and foreign standards regarding the calculation methods of water area dimensions，determine the preliminary design scheme for water area plan dimensions based on the experience，and carry out a ship maneuvering simulation test to verify the rationality of the design scheme.The results show that the water area layout of this project is reasonable，which can meet the requirement of ship navigation，and effectively reduce dredging volume and control engineering investment.