【Objective】 The trends of aridity index in Heihe river basin were investigated to provide basis for effective management of water resources and agricultural development. 【Method】 The aridity index (AI) was calculated as the ratio of potential evapotranspiration (ET_0) to precipitation (P). Daily ET_0 was estimated using the Penman-Monteith equation with the observed daily climate data for more than 50 years at 12 meteorological stations located in Heihe river basin and 3 stations out of it.The trends and spatial distribution of precipitation,ET_0 and aridity index in different time scales were detected by Mann-Kendall test and interpolation tools in ArcGIS,and the Morlet wavelet analysis was used to analyze the multiple time scale characteristics of aridity index. 【Result】 The annual precipitation,ET_0 and aridity index of the north basin were different from those in south basin in recent 50 years.The average annual precipitation and ET_0 at most of the 12 stations in the basin had increasing trends with time,and had significantly increasing trends at Yeniugou and Tuole stations in the upstream.Aridity index had decreasing trends,but only significant at Yeniugou station,indicating that the basin tended to be wetter.The upstream aridity index had peak values during autumn and winter,while midstream and downstream had peak values during spring and winter.Annual aridity index fluctuated with long cycles of 19-23 and 14-15 years and short cycles of 7-10 and 4-5 years.The long period was identical to sunspot cycle,while the short ones were basically same with the frequency of atmospheric circulation and El Nino events.【Conclusion】 The study area was becoming wetter in recent years,and variations of annual aridity index presented short and long cycles.