This study aimed to reveal the spatial-temporal variation of evapotranspiration (ET) in the loess Plateau during the past two decades and clarify the effects of climate and underlying surface changes on ET. Based on the data of 295 meteorological stations on the Loess Plateau, PML_V2 ET products and MOD13A1 EVI products, this study used trend analysis and multiple regression statistical methods to study the spatial-temporal variation characteristics of ET from 2000 to 2018, and the relative contributions of rainfall, temperature, sunshine duration, vapor pressure difference, vegetation and non-vegetation underlying surface were also quantitatively evaluated. The results showed that:(1) From 2000 to 2018, the annual change rate of ET on the Loess Plateau was 4.47 mm/a. The ET increased significantly in Shanxi, Qinghai and northern Shaanxi Provinces, accounting for 62.8% of the total area. The contribution of different land covers to ET was forest, farmland, grassland and shrub in descending order. (2) The significant increase of vegetation was the leading factor of ET changes in the Loess Plateau, and its relative contribution rate was the largest (32.1%). The relative contribution rates of different meteorological factors to ET from high to low were rainfall (14.6%), saturated vapor pressure difference (13.2%), temperature (12.4%) and sunshine duration (10.0%). The influence of non-vegetation underlying surface change (such as soil and water conservation engineering measures) on ET change could not be ignored.(3) The effects of meteorological factors and enhanced vegetation index on ET of forest and grass cover were greater than that of farmland cover, while non vegetation underlying surface elements had a greater impact on ET of farmland cover. The research results are expected to provide scientific support for water resources consumption, restoration and sustainability evaluation of ecological construction in the Loess Plateau.