Abstract: Abstract: In this paper, we studied the influence of a thinning intensities on the species diversity of understory plants in Larix principis Plantation in Sai Hanba mechanical forest farm of ChengDe.the three different treatment of thinning intensities are 225 trees/hm2，525 trees/hm2 and 735 trees/hm2 。Based on the calculations of diversity such as Simpson index, Shannon-Wiener index and Gleason index，we analyzed the species diversity of understory plants in Pinus Larix principis Plantation. The results showed that thinning had altered the understory light conditions, improved the growth of plant species diversity。There were 1.1－1.4 times species in all of the thinned plantations more than those of in the control plantations. The richness and Diversity of plant species were higher in Larix principis Plantation with different thinning intensities than those in the control area。Thinning improved richness and Diversity of plant species and mid-thinning intensity group was the most obviously。The richness and Diversity plant specie of herb layers were more 10.7-11.5 times and2.3—4.8 times than shrub layers in the three different treatment of thinning intensities of Larix principis Plantation。Coverage and density increased with the increase of thinning intensities of forest, and the community vertical structure became much clearer for the heavy thinning group。