Multi-scale quantitative assessment of the relative role that climate change and human activities play in desertification is an important approach to clarifying the causes of desertification. In this study, a quantitative method was developed to assess the relative roles of climate change and human activities in desertification by selecting NPP as an indicator. The potential NPP and the difference between the potential and actual NPP were used to represent the impacts of climate and human factors on desertification. Based on this method, the relative roles that climate change and human activities play in desertification reversion and expansion in the Ordos Plateau were assessed at different spatial–temporal scales. The results revealed that increasing the spatial scale resulted in the area of the primary desertification process and its dominated driving process becoming more predominant at coarser scales from 1980 to 20000. For assessment at multi-temporal scales, climate change was the dominant factor inducing the desertification reversion from 1980 to 1990; however, human activities controlled the desertification reversion from 1990 to 2000 and 1980 to 2000. Assessment at longer temporal scales may average the characteristics when it is assessed at shorter scales. Therefore, scale-dependent characteristics must be considered when evaluating the causes of desertification.