We investigated the understory species composition and diversity and determined the dominant stand factors affecting the understory species diversity using typical sampling method in five plantations (BF: Cupressus funebrisPterocarya stenoptera forest; YN: Ginkgo bilobaPhoebe zhennan forest; GZ: Swida wilsonianaCinnamomum camphora forest; FQ: Pterocarya stenopteraAlnus cremastogyne forest; CB: Cupressus funebris forest) in Yunding Mountain, Chengdu, to provide scientific basis for local plantation management. The results showed that: (1) a total of 168 understory plants were recorded, belonging to 62 families and 130 genera. The number of families, genera and species in the shrub layer and herb layer of five plantations was the greatest in the GZ. (2) The number of dominant species in five different plantation shrub layer was 7, 4, 7, 6, 4, and the number of herb layer was 5, 4, 9, 9, 10, all of which were small. (3) The ShannonWiener diversity index (H), Simpson dominance index (H′), Richness index (D), Pielou evenness index (J_sw) of five plantations were all expressed as: the herb layer > the shrub layer, the D of the BF and GZ was slightly higher in the shrub layer. The H, H′, and D of the shrub layer were the highest in the GZ, and there were no significant differences in the J_sw of the shrub layer among different plantations. The H, D and J_SW of the herb layer were sized as the CB, FQ, GZ, BF and YN, but the D of the GZ was slightly higher than FQ. (4) There were no significant differences in the effects of six stand factors on the four species diversity indexes in the shrub layer. The main stand factors affecting the D and H of the herb layer were average tree height, average branch height, average diameter at breast height, average crown and stand density. Stand factors had no effects on the H′ and J_SW in the herb layer. Research suggested that, the effects of stand structure on species diversity of the herb layer were more significant. Average tree height, average branch height, average diameter at breast height, average crown width and stand density had significant effects on the diversity of the herb layer.