With the floodseason precipitation data at 3 sampling meteorological stations in Jianghuai Watershed area of Anhui Province from 1957 to 2008, the Z-index method was applied to judge the flood/drought grades in Jianghuai Watershed area, and it is found that the theoretical Z-index is not suited to Jianghuai Watershed area. In this case the theoretical boundary of Z-index should be adjusted, and there are two principles of adjustment: the first is slightly adjustment, and the second is that years of drought and flood are nearly equal. The new boundaries of Z-index for the grades of flood/drought are more suitable for the Jianghuai Watershed area. Therefore the modified Z-index can be well applied to Jianghuai Watershed area.