Based on acquiring rainflood curve of Zhanghe River Basin and analyzing cases of historical rainstorm flood disaster risk， the disaster critical area rainfall and flood risk water levels of the river basin are obtained: The Gumbel I extreme distribution is used to calculate the area rainfall of different return period, and D8 and Manning’s formulas are also used to calculate the submerged area and depth of areal precipitation from different recurrence interval by GIS rainstorm flood inundation model. According to disaster risk theory， the rainstorm flood disaster risk regionalization of Zhanghe River Basin is done. Meantime， a plot of rainstorm flood disaster risk zoning of Zhongxiang City in Hubei Province is performed， and compared with the actual disaster situation, the flood disaster areas and high risk areas are correspondent. The results show that the method can express flood disasters intuitively, and also reflect the flood disaster risk of the basin and the study areas. It has a clear physical significance.