By use of the monthly precipitation data of 83 sampling meteorological stations in the Northwest China from 1961 to 1990,the anomalous percentage of precipitation, the standard variable of precipitation and the Z-index were tested and compared with each other. Grades of flood/drought for Z-index were again determined, the modified Z-index was recognized to be optimum for differentiating station flood or drought in the Northwest China. On this basis, flood and drought differentiating indices were given for the Northwest China.