Targeting the effect of vegetation restoration on the water cycle in the upstream of Yellow River, based on the vegetation coverage, remote sensing and field observation data of the evapotranspiration in the the Beichuan River Basin, the evolution trend of evapotranspiration and the causes of its changes were analyzed to investigate the potential influencing path of the vegetation restoration in alpine mountainous area on the changes of evapotranspiration on the basin scale. The results show that the evapotranspiration decreased by 33.23 mm in Beichuan River Basin from 2000 to 2019 due to the continuous increase of the vegetation coverage. The reduction of evapotranspiration in July and August, which is the main vegetation growth season, dominated the annual reduction. During the period of June to September, 2019, the soil evapotranspiration in the forest was 207.38 mm, which was 157.53 mm lower than that outside the forest, this was also conducive to the decrease of evapotranspiration in the basin. Meteorological factors such as surface wind speed, which were greatly influenced by vegetation, played an important role in the reduction of the potential evapotranspiration strength in the basin. The variation trend of evapotranspiration in alpine mountainous area in the background of vegetation restoration was different from that in the Loess Plateau and the eastern region, which was associated with its cold climate characteristics and the dominant vegetation type (coniferous forest). Furthermore, it fully reflected the water conservation function of ecological conservation forest in the upstream of the Yellow River.