Drought is a major limitation to plant survival and growth, which also has adverse effects on revegetation and reforestation practice. Quercus variabilis Bl., a deciduous broad leaf tree, is an important forest species and timber resource, and is often used in cork production, tannin extraction, and edible fungi cultivation in China. Especially in semi-arid regions of Northwestern China, it plays a key role in soil remediation, erosion control, and in the economical and ecosystemic development. In order to exposit drought adaptive mechanisms of this species seedlings, we investigated the effects of drought stress on growth, dry mass production, physiological and biochemical responses of thin fine roots (0.0 < D ≤ 0.5mm) and thick fine roots (0.5 < D ≤ 2mm) of this species seedlings. Three-month-old seedlings were implemented a potted experiment for 120 days with control, light, moderate and severe drought treatment, respectively, in a temperature-controlled greenhous. Results showed that: compared to control, the number, root length, surface area, volume, and dry mass of fine roots increased at light drought stress, whereas these parameters decreased at moderate and severe drought stress. The number and length of thin fine roots were higher than those of thick fine roots, while the surface area, volume, and dry mass of thick fine roots were higher than those of thin fine roots. Drought induced a decrease in relative water content and vigor of fine roots, as well as a increase in malondialdehyde (MDA) content(P < 0.001). The relative water content and vigor of thick fine roots were higher than those of thin fine roots, but MDA content of thin fine roots was higher than that of thick fine roots. Compared to control, both the accumulations of osmolytes (including free proline, soluble sugar and soluble protein), and activities of antioxidant enzymes (including superoxide dismutase (SOD), catalase (CAT), peroxidase (POD) and ascorbate peroxidase (APX)) increased at light drought stress, while the activities of POD and APX declined at moderate drought stress. The content of soluble protein and activities of CAT, POD and APX all reduced at severe drought stress. Moreover, accumulations of osmolytes and activities of antioxidant enzymes of thin fine roots were significant lower than those of thick fine roots under four drought stress treatments. When drought strength exceed moderate drought stress, the reduction in root growth, dry mass, physiological and biochemical adaptability for Q. variabilis Bl. seedlings started to occur. And thin fine roots with shorter life were more sensitive than thick fine roots. The fact that there was no serious dehydration of roots and seedlings death even under severe drought treatment, indicates this species has strong drought tolerance ability and has a great potential for vegetation restoration and reforestation in semiarid regions.