Long-term water-energy balance is a major concern in hydrology and water resource management. Evapotranspiration is a key factor for achieving water-energy balance. In this study, we used a simple water and energy balance equation to compare the effects of precipitation and potential evapotranspiration on actual evapotranspiration – mathematically and theoretically. The results showed that, in Baiyangdian catchment, a 1 mm or 10% increase in precipitation would lead to a 0.51 mm or 6.6% in actual evapotranspiration, and a 1 mm or 10% increase in potential evapotranspiration would lead to a 0.14 mm or 3.4% in actual evapotranspiration. The regional differences in the 10 regions of China showed that the effects of climate on actual evapotranspiration were significantly influenced by the aridity index. The changes of potential evapotranspiration will lead to more changes in actual evapotranspiration in humid regions, and the changes of precipitation will lead to more changes in actual evapotranspiration in arid regions.