Abstract: In order to provide a theoretical basis for precision improvement of artificial forest quality, a 10-year-old Pinus armandii plantation in western Sichuan was chosen as the research object, the effects of different thinning intensities (thinning retention of 1800, 1350, and 900 trees per hectare) on the species composition, biodiversity and vegetation structure of understory plants, and soil nutrients was explored ater three years of thinning. The results showed that: (1) The greater the thinning intensity, the more species and quantity of undergrowth plants, and photophilic plants are the pioneer plants that enter the sample plot after thinning; (2) The diversity index of understory plants increased with the increase of thinning intensity, and the Pielou index, Margalef index, Shannon-Wiener index, and Simpson index of all thinning intensities were significantly different from those of the CK (P<0.05). The Margalef index and Shannon Wiener index of thinning intensities M1, M3 were significantly different among different treatments (P<0.05); (3) The coverage of understory plants in all thinning treatments was significantly different from that of the CK (P<0.05), and there was also significant difference between M1 and M3. (4) There was significant difference in soil organic matter content between all thinning treatments and the control (P<0.05), and showing a significant difference between M3 and M1& M2 (P<0.05). Soil organic matter content was also significantly positively correlated with plant diversity index (P<0.05). The research indicates that from the short-term effect, the thinning intensity of 900-1350 trees per hectare is in favor of enhancing the understory plants diversity and soil nutrient of P. armandii plantation.