The Z index is widely used in flood statistics. Taking into account the temporal and spatial characteristics of flood disasters in small drainage areas, the 10day continuous rainfall proved to be more realistic and reasonable for calculating Z index. Through determining the flood disaster index of a single station and then compared with the traditional index, it is found that the new method can make up the deficiencies of the traditional one and describe the strength and starting and ending times of flood disasters. The flood disaster index computed by means of the new method can well fit the historical flood disaster cases over small drainage areas