Abstract: In order to study the effect of different thinning intensity on biodiversity of undergrowth in Picea asperata plantations, the representative samplings(20a, 30a, 40a) were chosen for different thinning modes including un-thinning (as the CK), mild thinning and moderate thinning in Miyaluo area, the sub-alpine region of western Sichuan. One year after thinning, the biodiversity indexes of shrub layer and grass layer were measured and analyzed under P. asperata Mast. Plantations. The result showed that 1) the biodiversity of undergrowth in 20-year-old P. asperata. plantations was not significantly affected by thinning. 2) The moderate thinning reduced significantly Shannon-Weiner index of grass layer in 30-year and 40-year old plantations. 3)The mild thinning increased significantly Shannon-Weiner index and Simpson index of shrub layer of 40-year-old plantations. As a result, thinning had a kind of influence on biodiversity of undergrowth in plantations of 30-year-old and 40-year-old in a short period of time.