Dongsheng mining area in Inner Mongolia is located in the fragile ecological area. Under the background of global warming and open coal mining, investigation of the variation of surface dry-wet conditions in this region over the past decades is of great importance. Based on the potential evapotranspiration(E_0) calculated by using the Penman-Monteith method, the ratio of E_0 to precipitation was used as aridity index(phi) for the years 19572012. The temporal variation, frequency distribution and periodical characteristics were analyzed. And a total derivative method was used to attribute the change in surface dry-wet conditions. The results showed that the annual aridity index was 3.2 with an increase of 0.004 per year. There was a drying trend in summer, while other seasons trended to be wetting, but the trend of four seasons was not significant. Significant periodicities of 18 -25 and 7-12 years were examined, and the primary period was 12 years. The recurrence intervals of maximum and minimum E_0 were both more than 50 years. An abrupt increase occurred in 1998. Maximum temperature was the main climatic factor affecting the change in surface dry-wet conditions,followed by precipitation and wind speed.