Abstract: Base on flash flood survey results,taking a small basin in Zhaotong City as an example, two methods are adopted for calculating design flood, which are:hydro-geography and instantaneous unit hydrograph. The discriminations and the characteristic of two methods are studied in detail using comparative analysis. The results indicates that the hydro-geography method is simple,easy to use and less dependent on data; Therefore, is widely used locally as an empirical formula. However, due to the subjectivity selecting parameters,results are generally smaller for situation where the data is lacking. The instantaneous unit hydrograph has a clear physical definition; it depicts flood processes more meticulous. Therefore, the results have higher consistency with other literatures in the past. In conclusions, the method of instantaneous unit hydrograph is optimal.