We used a typical sampling to study the impact of four types of stand densities (A: 650 trees/hm²; B: 1100 trees/hm²; C: 1250 trees/hm²; and CK: 1650 trees/hm²) on the species diversity, biomass and soil physicochemical properties of understory plant and their correlation in Cunninghamia lanceolata plantations. The results were as follows. (1) The Shannon-Wiener index, Patrick richness index of the shrub layer, and the Shannon-Wiener index, Simpson index, Patrick richness index of the herb layer showed a double-peak distribution: initially decreased, then increased, and then finally decreased again. The turning point appeared at densities of both A and C. The Simpson index and Pielou evenness index of the shrub layer decreased with stand density. The minimum values of the four diversity indexes of shrub layer and herb layer appeared at the density CK. (2) The herb layer had more biomass than the shrub layer. The total biomass of herb layer decreased with the increase of stand density, and the amount of litter increased with the increase of density. (3) The trends of the relationship between both diversity and soil physiochemical characteristics and tree density were similar. The organic matter decreased with the increase of density, with the maximum value presented at the density of A. (4) The Shannon-Wiener index, Patrick richness index of the shrub layer were positively correlated with the total nitrogen, hydrolyzable nitrogen, available phosphorus and organic matter contents of topsoil. There was no significant correlation between species diversity of the herb layer with the physicochemical property. (5) The correlation between the indexes and biomass of shrub layer was strong. In conclusion, density of 650 trees/hm² could promote the growth of undergrowth vegetation, increase the species diversity, biomass and organic matter of undergrowth vegetation, improve the ability to maintain soil fertility, and contribute to the stable and sustainable development of cunninghama lanceolata forest.