In the past few decades, drought has been a frequent phenomenon in many parts of the world, especially in arid and semi-arid regions. Hence in order to better predict the development trend of drought in Inner Mongolia and formulate corresponding countermeasures for disaster prevention and mitigation, for this paper we calculated the aridity index according to the annual mean precipitation and temperature data of 109 stations covering 1961-2010. The study result indicated that the mean aridity degree was increasing from west to east and was mainly characterized by moderate aridity; from 1961 to 2010 the change of aridity has been decreasing while aridity has been increasing, especially after the abrupt changing point in 1995. Moreover, the decreasing area of aridity took up 82.6% while the increasing area only occupied 17.4% of the whole area. During a 50-year time scale, the annual mean aridity oscillation periodicity was at 49a, 25a, and7a, and Inner Mongolia was still at a stage of aridity that might last for a long time. Regarding different decades, the climate of Inner Mongolia was relatively humid in the 1980s; however, as time went by the area of moderate aridity degree increased while the humidity area decreased in the early 21st century. Therefore, it was clear that the climate was becoming drier year after year. From 1960 to 2010, contours lower than 30 shifted toward longitude direction and contours more than 30 moved in the direction of latitude.