Estimation of actual evapotranspiration (ET) and diagnosis of its controlling factors contribute to addressing water scarcity challenges in arid regions. For detecting impacts of vegetation, precipitation, and evaporation capacity on ET, a monthly ET model was proposed on the basis of the three factors and applied in the upstream and midstream of the Heihe River basin, northwest China, an arid and semi-arid basin. Parameter sensitivity analysis, partial correlation analysis, and factor analysis were used to assess the controlling factors of ET. Results demonstrated that the proposed ET model can reconstruct monthly ET processes with satisfying performances and precipitation, evaporation capacity and vegetation are the controlling factors of ET in the study areas. The proposed ET model could be applicable with the three controlling factors to estimate ET in arid regions.