Traditional reasoning formula methods and instantaneous unit hydrograph methods are difficult to reflect geomorphological features and land use changes in design flood calculation. To address this issue, a new design flood flow concentration calculation method, namely the runoff curve number (SCS) geomorphologic general unit hydrograph method, was proposed by considering the influence of geomorphological features and land use changes on the time of flow concentration. This method was coupled with the SCS model to establish a complete flow generation and concentration model, and the Shangbu River Watershed in Hangzhou City was taken as an example to verify the method. The verification results show that compared to traditional reasoning formula method and instantaneous unit hydrograph methods, the SCS geomorphologic general unit hydrograph method is more reasonable for calculating design floods in small watersheds within urbanized areas, as it better reflects the watershed’s geomorphological features and adapts to land use changes. Scenario analysis further reveals that the changes in underlying surface conditions caused by urbanization will lead to an increase in design flood volume and flood peak.