Abstract: Taking a 25-year old Chinese fir (Cunninghamia lanceolata) plantation as the object, this paper studied the effects of thinning on the biomass, morphological traits, and nitrogen concentration of the first five orders roots. With the increase of root order (from the first to the fifth order), there was a significant increase in the fine-root biomass, diameter, and tissue density, and a significantly decrease in the specific root length (SRL), root length density (RLD), and root number (RN). Thinning increased the biomass, RLD, and RN of the first and second orders roots as well as the tissue density of the first, third, fourth, and fifth orders roots significantly, but had no effects on the SRL and nitrogen concentration of each order root. In contrast, thinning decreased the diameter of the first, third, and fourth orders roots significantly. The diameter of the second order roots was obviously smaller in surface (0-10 cm) soil than in subsurface (10-20 cm) soil, while the RLD of the first three orders roots and the RN and nitrogen concentration of the first two orders roots were larger in surface soil than in subsurface soil. The interaction of thinning and soil layer only decreased the diameter of the first two orders roots. It was suggested that the fine-root biomass and morphological traits of Chinese fir were closely related to the vegetation growth and regeneration after thinning.