By using of Penman formula, the authors calculated the evapotranspiration since 1980s, and analyzed the variation tendency of some climatic factors such as evapotranspiration, sunshine duration, air temperature, air saturation deficitetc, and focused on their impact to the evapotranspiration. The results show that the evapotranspiration is increasing annually with the speed of 3.25 mm over Yellow River upper stream area;while the sunshine duration which is considered as main impact factor, is increasing with the speed of 3.6 h/a;air temperature is appearing the increasing tendency, and its climatic tendency is 0.4℃/10 a; air saturation deficit is increasing with 0.02 hPa/a. Therefore, we could think that the evapotranspiration is greatly increasing as increment of sunshine duration and air temperature deficit over Yellow River upper stream area. But the increment of evapotranspiration and decrement of precipitation amount impact directly on the decrement of flow curve and expanding of grassland' s desertization over Yellow River upper stream.