Abstract: The influence of stand density on species composition and diversity of associated plants and understory vegetation, as well as natural capacity, is the basis of studying the relationship between forest structure and function, community stability, genetic conservation and natural forests management technology. The species and quantity of all trees, shrubs and herbaceous plants in the standard plot of different stand densities were recorded by the standard plot survey method, and the diversity indexes were calculated and analyzed. At the same time, the number of natural regeneration seedlings under different density stands was recorded. The results showed that there were nine species of associated trees and 17 species of understory shrubs and herbaceous plants. The sensitivity of tree, shrub and herb species richness to the change of stand density was basically the same from the analysis of different cross degrees. From the range of species richness, trees, shrubs and herbs were more sensitive to the change of stand density. The response of understory vegetation diversity to the continuous increase of stand density was reflected as a non-single change trend. With the increase of stand density, the diversity index of stand vegetation increased at first and then decreased, showing a bimodal response pattern. In the study area, the natural spruce stands maintained a high level of understory species diversity within the density range of 400-499 ·hm^-2 trees. The natural regeneration ability of Picea brachytyla natural forest was poor, and natural regeneration seedlings were found in only four density stands. Therefore, in the process of genetic conservation, the stand density should be adjusted reasonably to promote natural regeneration of Picea brachytyla natural forest.