Reference crop evapotranspiration(ET₀)is wide lyconsidered to be one of the best means to evaluate agriculture water use,which is of great importance for crop production. The agriculture in the arid region of northwest China has suffered from severe water shortage and imbalance between water quantity and land area. Under the background of global warming, it is necessary to evaluate the spatiotemporal characteristics of ET₀as it is sensitive to climate change. Observed daily data during 1961-2011 from 152 meteorological stations, including temperature, relative humidity, wind speed and sunshine hours,are used to calculate the ET₀ based on Penman-Monteith equation.Mann-Kendall, Pettitt and Inverse Distance Weighted Interpolation are used to analyze the spatiotemporal characteristics of ET₀.Results show that the annual mean ET₀is1100.4 mm/a with obvious spatial variations. Values in the south region of the Loess Plateau are the smallest, while those in a southeast-northwest strip beside the Loess Plateau are the greatest.ET₀ shows a insignificant upward trend during 1961-2011 for the whole region,and the annual increase is 0.45 mm/a; however, significant upward and downward trends are detected for 64 and 36stations, respectively.ET₀has occurred abrupt changes from 1970s to 1990s;however, the years occurring abrupt changes in the eastern part are later than those in the western part. Sensitive coefficients and contribution rates are calculated to analyze the reasons of the spatiotemporal pattern for ET₀.Results show that the spatiotemporal characteristics of ET₀are from the combined effects of climatic factors. Relative humidity and maximum temperature is the most sensitive factors for ET₀; however, wind speed and maximum temperature have the greatest contribution rates for the changes of ET₀ during the past 50 years.The changes of ET₀ in the arid region of northwest China have transformed from downward trend to upward trend since 2000s, which will exacerbate the water shortage of this region. Therefore, more attentions should be paid to it and some countermeasures should be planned.