Ecosystem water use efficiency (WUE) is a key proxy to characterize the coupling of carbon and water cycle. However, the temporal and spatial characteristics of WUE on the Loess Plateau and its leading factors under the background of climate change and the project of returning farmland to forest and grassland are still unclear. The remote sensing driven ecosystem process model (BEPS) was used to simulate gross primary productivity (GPP) and evapotranspiration (ET) of the Loess Plateau from 2001 to 2020. The contribution of climate and vegetation to WUE changes (WUE=GPP/ET) was quantitatively analyzed by combining sensitivity test and multi-control factor solution. Results showed that: (1) During 2001-2020, GPP and ET increased significantly at 12.9 gC m^-2 a^-1 and 3.7 mm/a, respectively, and led the significant WUE increase (0.021 gC mm^-1 m^-2 a^-1). (2) In the past 20 years, 80.12% regions of the Loess Plateau showed significant increased LAI (with regional growth rate of 0.014 m² m^-2 a^-1), while the change of climate factors was not significant. (3) Vegetation and climate factors exhibited positive and negative contribution to WUE change, respectively. As the main influencing factor, vegetation dominated the WUE change in 86.74% of the Loess Plateau.