Understory plants and soil characteristics of Acacia auriculiformis plantations with three densities were studied.Results showed that the total coverage degrees of understory plants were in the order of lowdensity stand(1 667trees/hm²)> middle-density stand(4 444trees/hm²)> high-density stand(10 000trees/hm²).The total biomass of understory plants were in the order of low density stand> middle-density stand>high-density stand,and the shrub biomass of low-density stand and the grass biomass of middledensity stand were the greatest among the three stands.In the low,middle and high density stands,Simpson diversity indexes for shrub layer were 0.679,0.935and 0.708,and for grass layer,0.904,1.228and1.064;Shannon-Wiener diversity indexes for shrub layer were 1.657,0.535and 1.171and for grass layer,0.904,1.228and 1.064;and Pielou evenness indexes for shrub layer were 0.691,0.333and 0.654,and for grass layer,0.504,0.886and 0.594,respectively.Except the available P,the soil characteristics of the low and middle density stands were better than those of high-density stand.