Based on the meteorological data,such as monthly precipitation,mean temperature,mean wind speed,from 96 representative stations in Inner Mongolia from 1971 through 2010,the potential evapotranspiration and aridity index were calculated with Penman-Monteith model. The characteristics of spatial-temporal of aridity in Inner Mongolia were discussed by using mathematical statistics of climate tendency rate and moving average. The consequences of aridity change were discussed. The relations between aridity index and water resources, precipitation,sunshine duration,relative humidity and meteorological factors were discussed also in this paper. The results show that(1)from 1971 through 2010,in Inner Mongolia the aridity index had decreased with a decreasing rate of -0.003·(10a)~(-1);aridity index had great spatial differences,gradually decreasing from east to west;a high center was in the northeast of Inner Mongolia(Northeast of Daxing'an Mountains)and a low center took place in northern Badain Jaran Desert;(2)since the 1990s,under the background of global warming,aridity index has decreased obviously in Inner Mongolia,especially in the east;aridity type has changed from wet to dry significantly;climate has been characterized by dry and warm due to reduction of precipitation and regional warming;(3)precipitation was the primary factor to the climate change(with correlation coefficient r>0.9), followed by sunshine duration,relative humidity. Water resources have decreased sharply in Inner Mongolia during the past decade,which may have something to do with the change in aridity conditions.