•We analyzed change trends of the climate variables ET0 and AI in an arid region of China over the past 50years.•We identified the sensitivity of ET0 and AI to the main climate variables.•We quantified the contributions of climate variables to the changes in ET0 and AI.•Contribution of wind speed decline to decrease of ET0 is larger than other climate variables.•The increase of precipitation contributes more than the decrease of ET0 to decrease of AI. Temporal variations in reference evapotranspiration (ET0) and aridity index (AI) were comprehensively investigated for 23 meteorological stations during 1955–2008 in the northwest China. The quantitative contributions of the major meteorological variables to the ET0 and AI trends were evaluated and the possible causes were also investigated. The results showed that in the past 50years annual temperature, humidity and precipitation had significant increasing trends with time, and wind speed and radiation had decreasing trends. ET0 had a significant decreasing trend with an averagely value of about 3mm per year, and AI had also witnessed a decreasing trend; For ET0, wind speed was the most sensitive meteorological variable, followed by relative humidity, temperature and radiation and for AI, precipitation was the most sensitive meteorological variable. The contribution of wind speed to the decrease of ET0 is more than that of other meteorological variables. The increase of precipitation contributes more than the decrease of ET0 to decrease of AI in past 50years. This study provides an understanding of the effect of recent climate change on drought in arid northwest China.