In the existing code, the flood frequency standard is used to determine the port designed high water stage. The mountain river has the characteristics of short lasting time for the flood course, high perk value and sharp -thin peak shape, and the flow pattern is rather bad. The wharf’s designed high stage, which is determined by the flood frequency standard, greatly exceeds the prohibited navigable stage in all place, and it is difficult to meet the demand for ships’entering, leaving and berthing at the port. The top altitudes of the port structures determined according to the designed stage increases the cost, but the increased working time is only about 3 days. Through analyzing the model test results of the typical engineering and the hydrograph statistics, we discuss the feasibility and rationality of adopting the guarantee of navigation duration in the high level to determine the port designed high stage.