Evapotranspiration (ET) is mainly composed of evaporation (E) and transpiration (T), however it is often difficult to separate E and T from ET in a terrestrial ecosystem. To better understand the effects of E and T on ET of alpine meadow in the Three-River Source Region (TRSR) (89°24'—102°23'E, 31°39'—36°16'N) in Qinghai Province, lysimeter and micrometeorological measurements were used to quantitatively study the variations of ET, E and T, and effects of vegetation and environmental factors on evapotranspiration processes. The results showed that the ET in 2017 and 2018 was 467.7 mm and 479.2 mm, respectively, and ET during the growing season (May-September) accounted for about 72% of annual total ET, while the contribution of E to ET (56%) was greater than that of T (44%). Furthermore, more than 90% of annual precipitation was returned to the atmosphere through the ET processes (ET/P > 90%). Based on the results from lysimeters with different vegetation coverages during the growing season, it was found that ET increased with the decreasing of vegetation coverage. The results of stepwise regression analysis indicated that net radiation (R_n) was the most important factor driving ecosystem ET, E and T; in addition, the response of E to vapor pressure deficit (VPD) was more sensitive than T, while the air temperature (T_a) was more significantly associated with T in comparison to the E. The effect of soil water content (SWC₅) on ET was relatively small maybe due to the precipitation was relatively abundant in this alpine meadow. The results suggested that the degradation of alpine meadow would lead to the increase of E, as a result more water is lost from the ecosystem through evapotranspiration.