Surface evapotranspiration is a key process for measuring the effect of climate change on the regional hydrothermal cycle. However, the quantitative impact of vegetation on evapotranspiration is difficult to measure due to the effects of climate change and human activities. This study used time series analyses to describe the vegetation changes from 2001 to 2019. A structural equation model (SEM) for the Haihe River basin was then developed using remote sensing data and meteorological forcing data. The purpose was to separate the impact of climate change from the impact of vegetation change on the evapotranspiration. The results show that the vegetation leaf area index (LAI) increases significantly during the study period (0.014 (m²/m²)/a) and that the vegetation has a significantly greater influence on evapotranspiration than climate change. Climate factors indirectly exert a strong influence on evapotranspiration through vegetation. This study provides a basis for the assessment and integrated management of water resources in this area.