Concentrated localized torrential rains due to global warming and climate change have resulted in much water damage each year. GIS is used as a tool for predicting the peak-outflows caused by these regional torrential rains in mountainous rivers. However, the research of the resolution of the data is limited, and most of approaches are about hydrological geographic. This paper estimates the flood discharge needed for decision of standard rainfall of automatic rainfall warning system by using GIS with GcIUH model, and establishes the criteria of flash flood warning. It also has analyzed the terrain in river basin, extracted the morphological characteristics parameters of water shed such as stream width, channel slope, channel length, shape factor, and GcIUH parameters, and analyzed the relationship between them.