Evapotranspiration plays an important role in the process of terrestrial hydrothermal cycle. Due to the unique geographical environment of the Qinghai-Tibet Plateau (QTP), the research on evapotranspiration has received great attention. Based on the meteorological grid dataset with a resolution of 0.05° and MODIS remote sensing data, the daily evapotranspiration was estimated on the QTP from 2002 to 2019 by using the complementary relationship method. The spatiotemporal variations of evapotranspiration were analyzed, as well as the relationship with permafrost types and meteorological elements. The results showed that: (1) the estimated evapotranspiration by the complementary relationship method was reliable on QTP. (2) The average evapotranspiration on QTP from 2002 to 2019 was about 371.54 mm/a. It had obvious seasonal differences, and mainly concentrated in summer. The spatial distribution characteristic of the evapotranspiration showed a deceasing trend from southeast to northwest on QTP. The evapotranspiration increased in the east of the QTP, but decreased in the west. The stability of the evapotranspiration was significantly different, and the fluctuation intensity of the evapotranspiration showed a decreasing trend from southeast to northwest. (3) The evapotranspiration was closely related to the spatial distribution of permafrost types, that was, the more stable the hydrothermal state of the permafrost, the smaller the evapotranspiration. (4) The influence of meteorological elements on the spatiotemporal changes of evapotranspiration had obvious differences. Evapotranspiration was most sensitive to net radiation, while relative changes in wind speed contributed the most to evapotranspiration variation. The research results improved the response mechanism of the QTP's water cycle under the background of climate change, and provided a basis for the formulation of regional ecological and environmental protection measures.