Pinus tabulaeformis plantations play an important role in local water and soil conservation and ecological protection in the Loess Plateau. Because of the large surface area, short life span, and rapid turnover, fine root systems were considered an important part in material cycling, energy transportation, and soil and water conservation of the forest ecosystem. This study was conducted in a middle-aged P. tabulaeformis plantation, located in the Huanglong Mountains, south of Loess Plateau, China. Thinning was performed on three occasions in 2008 following afforestation: light thinning, medium thinning, and heavy thinning. Three replicates of thinned and un-thinned treatments (20 m×30 m) were selected in our research. Basic data of fine root biomass and morphological characteristics were collected from three soil depths (0-20 cm, 20-40 cm, and 40-60 cm) via root drilling methods. The results showed that fine root biomass increased with increasing thinning intensity up to the medium thinning treatment and then decreased in the heavy thinning treatment. Fine root biomass in the 0-20cm soil layer decreased significantly (P < 0.05) under heavy thinning intensity compare with other treatments. The proportions of fine root biomass in the 20-40-cm and 40-60-cm soil layers increased with increasing thinning intensity. The differences in root length density and root surface area density between different thinning intensities and different soil layers were significant (P < 0.05), and exhibited a similar trend as the fine root biomass. With increased thinning intensity, specific root length and specific surface area of fine roots increased, and the differences were significant under heavy thinning (P < 0.05). Light and medium thinning had a significant effect on finest diameter fine roots (0-1.0 mm), but no significant effect on fine roots (1.0-2.0 mm) (P < 0.05), whereas heavy thinning had a significant effect on all roots (0-20 cm) (P < 0.05). Fine roots may be distributed in the deep soil layer after thinning. Compared with fine roots, the finest ones were more sensitive to different thinning intensities. In this study, fine root biomass (1022.43 g/m²), along with the root length density and root surface area, peaked at the medium thinning intensity, which could be the appropriate silviculture practice for tree growth.