【Objective】 To study effects of stand density on understory species diversity and soil nutrients in Chinese fir plantations, a field survey was carried out in a Chinese fir plantation, 38 years old, varying in stand density in Dagang Mountain, Jiangxi Province. 【Method】 Five plots of woodlands different in stand density were delineated for investigation of understory species therein, and soil samples were collected from each plot for analysis of soil physical and chemical properties and soil enzyme activities. And One-way ANOVA analysis and correlation analysis and principal component analysis of the obtained data were performed for determination of relationships of understory species diversity and soil properties with stand density. Besides. principal component analysis and factor analysis were conducted to calculate scores of various index factors of the plantation, in an attempt to screen out a suitable density for growth of Chinese fir trees in plantations. 【Result】 Results show that there were 121 species of understory plants found under the Chinese fir trees in the plantation. The shrub layer was dominated by Maesa japonica, while the herb layer was by ferns, such as Diplazium and Dryopteris fuscipes. The two layers both were the highest in diversity in the plot, 5 000 trees hm^-2 in initial density. The variation of soil nutrients did not differ much with soil layer, but did with stand density. All the soil nutrients, except pH, organic carbon and celllase, were the highest in the plots either the highest or the lowest in stand density and obviously varied with stand density. Soil nutrients in the 0~20 cm soil layer were more closely related to herbaceous diversity, while soil nutrients in the 20~40 cm soil layer were more to shrub diversity. The factors of pH, total nitrogen, alkalytic nitrogen, available phosphorus were the most closely related to the species diversity index of the understory. The principal component analysis shows that the plot 6 667 trees hm-2 in initial stand density was the highest in comprehensive score (1.17), and followed by the plots 3 333 trees hm^-2 (0.93), 5 000 trees hm^-2 (0.28), 1 667 trees hm^-2 (0.12) and 10 000 trees hm^-2 (-2.49). obviously the first two were significantly higher than the other three in score. 【Conclusion】The research on plant diversity index and soil physicochemical properties reveals that the initial density of 5 000 trees hm^-2 is good to stability of understory species diversity, but not so to accumulation of soil nutrients. The principal component analysis shows that stand density, either too high or too low, is detrimental to the development of soil physical and chemical properties and plant diversity, especially when the density is too high, its damage to forest land is enormous, exhibiting a "hump model", which indicates the initial densities of 6 667 trees hm^-2 and 3 333 trees hm^-2 are more suitable to the development of soil physical-chemical properties and plant diversity. The "Hump model" shows that taking into comprehensive account, the co-existence of different stand densities in a plantation may better maintain soil properties, increase plant diversity in the ecosystem and improve the ecological environment, thus contributing to multi-purpose cultivation and sustainable development of of Chinese fir plantations.