The paper used climatic trend rate, inverse distance weighted, and Mann-Kendall and wavelet analysis method to analyze the evolution of dry index so as to provide scientific basis for the agricultural development,protection of ecological environment and dispatch operation of cascade hydropower in Datong River basin.The results showed that：there is unremarkable trend of becoming humid in surface of the basin, the annual aridity index is decreased by 0.05 per ten years with, but there is anti-persistence.The climate of the basin is dry in south and wet in north,and dry in east and wet in west on spatial distribution.The status of dry and wet presents more clear pattern of banded alternation; the climate of four seasons also shows the tendency of wet.The humidified trend is more obvious in spring and winter.The aridity indexes in spring and winter are decreased by 0.049 and 3.79 per ten years respectively. However, only the trend in winter passed significant test,and the wet trend has anti-durative.The temporal distribution of season dry index is uneven. There is obvious regional differences in the dry and wet change of season climate; the periods of annual aridity index are 7,17 and 29 years;the cycles of aridity index in spring are 4,13 and 28 years,and that in summer are 8,17 and 30 years,that in.autumn are 5,17 and 26 years while that in winter are 5,9 and 29 years.