The design flood in Liuxihe reservoir basin in Guangdong province was computed through three approaches: long-term discharge data, inference formula for Guangdong province, and WetSpa distributed hydrological model (for water and energy transfer between soil, plants and atmosphere based on GIS). The computation result based on WetSpa model was closer to that on discharge data than that on inference formula, which showed that computation method based on WetSpa model is practicable. This method needs only digital elevation model (DEM), land use and soil type of the basin as well as a small amount of rainfall and flood data. The DEM, land use and soil data could be downloaded from the Internet. The WetSpa model provides a new approach of calculating design flood for areas in lack of data.