In order to find a reasonable density to maintain a stable community structure and improve species diversity, we studied the effects of different stand densities on community structure and species diversity of Eucalyptus robusta plantation in Puxing Town, Xinjin County, Sichuan Province, using typical sampling method, setting 3 plots in each stand density (A. 625 trees·hm^-2; B. 750 trees·hm^-2; C. 875 trees·hm^-2; D. 1 000 trees·hm^-2; E. 1 125 trees·hm^-2). The results showed that: (1) a total of 96 plants were recorded, belonging to 51 families and 79 genera. The number of species in the vegetation of every kind of stand density arranged from greatest to least was: the herb layer, the shrub layer, and the tree layer. When the density was 1 000 trees·hm^-2, there were 6 species in the tree layer, which was the maximum. And the maximum was 17 species in the shrub layer when the densities were 750 and 875 trees·hm^-2 , there were 32 species in the herb layer under the stand density 625 trees·hm^-2.（2） In general, the species richness index D, ShannnonWiener index H, Simpson index H′ and Pielou uniformity index Jsw of the E. robusta plantation community in all kinds of stand densities were all expressed as: herb layer＞shrub layer＞tree layer. The maximum of the D and H′, H, J_sw value of the tree layer was respectively from the density C, A, B; each index of the shrub and herb layer had the maximums respectively at the density 750 and 625 trees·hm^-2. (3) Seen from the community structure, the diameter class of the five kinds of stand densities showed a single peak type distribution, and the trees were mostly distributed in the middle diameter classes(7.0≤d＜23.0 cm); the height class was different from the diameter class, the trees in the density of A, B, C and D, E were concentrated respectively in the height classes Ⅱ to Ⅴ(6.0≤h＜18.0 m), Ⅲ to Ⅴ(9.0≤h＜18.0 m), Ⅱ to Ⅲ(6.0≤h＜12.0 m), Ⅳ to Ⅴ(12.0≤h＜18.0 m). (4) The dominant tree species varied among different layers. It was E. robusta in the tree layer, and the important value of Broussonetia papyrifera in each density was second only to E. robusta in general. In the shrub layer, E. robusta seedlings only appeared in density E, and the dominant position of B. papyrifera in each density was significant. In density B-D, both Alangium chinense and Ligustrum lucidum existed, in which they grew well with the important value of 0.118 4 and 0.183 2 under density B, having great potential for renewal. As the dominant species of herb layer, Artemisia carvifolia and Arthraxon hispidus both grew under the five kinds of stand densities. The results showed that the stand density 750 trees·hm^-2 was the optimum for the E. robusta plantation, which was more conducive to the stable and sustainable development of community structure and species diversity of E. robusta plantation in this area.