Evapotranspiration (ET) is an important component of terrestrial ecosystem water balance. To examine the ET characteristics of degraded meadow in the Three-River Source Region (TRSR), we quantitatively studied the variation of ecosystem ET and the effect of environmental and biological factors on ET with eddy covariance and micrometeorological measurements from 2016 to 2017. To further characterize the ET at different stages, each year was divided into three periods based on soil temperature: i.e. frozen period, freeze-thaw period, and thawed period. The growing season was further defined as from May to September within the thawed period according to plant growth status. We also explored the influence of soil freeze-thaw on annual ET. The results showed that the annual precipitation in 2016 and 2017 was 451.8 mm and 442.3 mm, respectively, but the amount of annual ET in 2017 (485.6 mm) was obviously higher than that in 2016 (428.6 mm). The seasonal variation of ET showed a similar pattern for two years, with the peak value in July-August and the lowest value in December or January, and the amount of ET in the growing season accounted for about 72% and 73% of annual ET, respectively. The length of frozen and freeze-thaw periods in 2017 was 8 days shorter than those in 2016, while the amount of ET during the thawed period in 2017 was 63.1 mm more than that in 2016, of which the growing reason was 36.3 mm higher. The average daily rate of the ET was 1.81 and 1.97 mm/d in thawed period, 2.05 mm/d and 2.29 mm/d in the growing season, 0.97 and 0.73 mm/d in the freeze-thaw period, and the lowest rate was in frozen periods with only 0.27 mm/d and 0.33 mm/d for 2016 and 2017, respectively. The results of multiple stepwise regression analysis showed that net radiation (R_n) had the greatest impact on ET, and then was the temperature (T_a) and soil water content (SWC₅) in 2016, while ET was mainly controlled by R_n and T_a in 2017. The canopy conductance (g_c) and decoupling coefficient (Ω) in the growing season and thawed period were obviously higher than those in other two periods, and the value of both g_c and Ω in 2017 was higher than those of the same period in 2016. Our study suggested that the obvious variations in ET for each period and year might be caused by the change of freeze-thaw cycle occurrence and different vegetation status due to different radiation and temperature. The results of this study can provide a reference for comprehensively exploring the characteristics of evapotranspiration in the TRSR.