With the data of monthly meteorological drought integrated index (Ci) of 40 meteorological stations in the east region of Northwest China from March to May in 1982-2005 and the 500 hPa monthly mean geopotential height field in the corresponding period from the dynamical extended prediction provided by the National Climate Center, the application and test have been done by the MOS method in the aspect of explanation and reanalysis of numerical forecasting products. The result showed that the average accuracy to Ci predicted by the original equation was only 66% in spring, but the corrected average accuracy reached 77%, and increased by 11% according to sequence analysis of 40 stations, with a maximum rise of 15% in March, and a minimum rise of 8% in May. Particularly, the increment of the minimum value was the greatest with a 25% increment in average.