Reference evapotranspiration is an important component of the thermal balance and water budget during the water cycle of the earth surface; it is subjected to climate change directly. The research on reference evapotranspiration changes could help us understanding the characteristics of climate change and regional water cycle. Based on the observation data of temperature, wind, vapor pressure and sunshine hours collected from 124 weather stations from 1961 to 2007, the reference evapotranspiration in Northeast China was calculated by using Penman-Montieth model. Spatial-temporal characteristics of reference evapotranspiration and its causes in Northeast China were evaluated by space and time series analysis method. The results showed the reference evapotranspiration has continuously decreased at a rate of 4.4 mm/decade in Northeast China, but the annual mean temperature has increased at a rate of 0.38℃/decade. In the 1960s to the 1990s, the trend of reference evapotranspiration declined, and the trend of rising was significant in the 21st century. The average of spring and summer reference evapotranspiration in Northeast China decreased at a rate of -4.2 mm/10 a and-2.2 mm/10 a. There was a slight increase in reference evapotranspiration in autumn and winter at the rate of 0.3 mm/10 a and 1.6 mm/10 a in the recent 47 years. For spatial distributing characteristics of the average annual reference evapotranspiration, two-thirds of the region show decreasing trend especially in Chaoyang region of Liaoning Province, and one-third of the region show increasing trend especially in the south of Liaoning and the east of Jilin. The reference evapotranspiration was increased suddenly in spring from 1969, in summer from 1963, in autumn from 1963 and in winter from 1990. The correlation analysis of meteorological factors indicated that the most important factors which could influencing the reference evapotranspiration were sunshine duration and wind speed. Besides the meteorological factors influencing reference evapotranspiration, great concern has been given to the geographical location, land use, vegetation coverage and human activity.