There are hilly regions in coastal hilly cities.The flood discharges rapidly due to the upstream hilly areas.What's more,the drainage of rivers may be influenced by the backwater effect of the tide,which makes the flood disasters be more frequent in these areas.Urbanization has also changed the runoff characteristics of original underlying surface.Neither the calculation method of design flood of natural river nor the calculation method of city drainage can be used in these areas.In view of the characteristics of small watershed in hilly areas of coastal cities,the paper analyzed the difference between the calculation formula of city municipal drainage,water conservancy reasoning formula and unsteady flow calculation method.It proposed the calculation method of design flood in small watershed which was based on reasoning formula,pipe junction and unsteady flow calculation method.The paper took Zhewang-Shiqiao district of Ganyu county as case study and used the above three methods to calculate design flood.The results gotten by unsteady flow calculation method is smaller than that by reasonning formular.This method is more in line with the characteristics of flood in coastal hilly region.The result can provide a basis for city construction and river planning.