Based on a numerical study on the dust storm process occurred from March 18 to March 22, 2002, the influence of the dust source area in Inner Mongolia on the dust storm extending process was conducted. The results show that the degradation of the desert steppe in Inner Mongolia was a reason causing the frequent appearing of dust storm in northwest China. Owing to the distribution characteristic of terrain height in sand source area and the change of the geostrophic wind with height, two isolated preponderant transporting zones often appear in the dust particle transporting process. One lies in the mid\|troposphere while the other closely upon the surface. The transportation speed, distance and their impact area and time are different in the process. The raising dust in Inner Mongolia was transported mainly through the zone upon the surface. Its transportation in troposphere became less and less while the dust storm extended southward and eastward.