Abstract: Larix principis-rupprechtii plantations are major public welfare forests in Taiyue Mountain, North China. However, the stand quality of L. principis-rupprechtii plantations is limited due to the high stand density. Density adjustment is therefore of great significance in improving the quality of plantations, increasing the understory species diversity and maintaining ecosystem functions. In order to investigate the influence of density adjustment on canopy structure, light environment and species diversity of understory vegetation, a thinning experiment was conducted in a 35-year-old L. principis-rupprechtii plantation with heavy (50%), medium (30%) and light (20%) intensities and unthinned plots as control in Taiyue Mountain. The results showed that: 1) density adjustment had significant effects on the forest canopy structure and light environment of understory. The spatial heterogeneity of canopy structure was increased following the density adjustment. After thinning treatments at 20%, 30% and 50% intensities, the canopy leaf area index (LAI) was reduced by 18.25%, 25.76% and 37.50%, respectively, compared to the control, while the openness was increased by 17.45%, 17.45% and 17.45%, respectively. Photosynthetic photon flux density (PPFD) was 1.3, 1.4 and 2.1 times that of the control, respectively. 2) The number of understory species and their quantities were increased by changing the structural characteristics of the understory community. The highest species diversity and the most complex structure appeared in the 30% thinning treatment. 3) Correlation analysis and CCA sorting analysis showed a significant correlation between the canopy structure and diversity of understory vegetation. The diversity of herb layer species had a significantly negative correlation with LAI, which was the main factor affecting species composition in the herb layer.