Abstract: Based on the modified Penman-Monteith ( P-M ) model, the land surface reference evapotranspiration in the source area of the Yellow River from 1980 to 2020 was calculated by using the meteorological station observation data and the land-atmosphere water-heat exchange observation data. The temporal and spatial evolution characteristics of evapotranspiration in the source area of the Yellow River were analyzed, and the reasons affecting the change of evapotranspiration in the source area of the Yellow River were discussed.The results show: (1) The modified P-M model can accurately estimate the reference evapotranspiration in the Source Region of the Yellow River, and the correlation coefficient with the actual observation is above 0.85. (2) The evapotranspiration of the Source Region of the Yellow River shows an increasing trend, but it shows a significantly decreasing trend in the mid-1980s and mid-1990s. In recent years, the evapotranspiration in the central and western regions shows a decreasing trend, while the evapotranspiration in the eastern region shows an increasing trend. (3) The annual evapotranspiration of the Source Region of the Yellow River shows a decreasing trend from east to west,, with 473.5~516.0mm, 437.6~473.5mm and 386.3~437.6mm in the east, central and western regions, respectively. Evapotranspiration was significantly different in four seasons, with the largest in summer, followed by spring and autumn, and the smallest in winter. (4) The evapotranspiration increases with the increase of temperature, wind speed and sunshine hours, and decreases with relative humidity and precipitation.