To better understand the contribution of understory to biodiversity of a forest ecosystem, we examined the understory in stands of: (1) a Phyllostachys edulis (Moso bamboo) plantation, (2) a Cunninghamia lanceolata (Chinese fir) plantation and (3) a natural evergreen and deciduous broad-leaved mixed forest (natural mixed forest) in Mt. Mao’er in southern China, and the distribution and diversity of understory in relation to environmental conditions (overstory structure, soil nutrients, and gap light level). Soil samples were taken and analyzed in a laboratory for pH, organic matter (SOM), and total nitrogen (TN). Hemispherical photographs were taken using a Panasonic DMC-LX5 digital camera and analyzed using Gap Light Analyzer 2.0 software. Statistical methods, such as multi-response permutation procedure (MRPP), detrended correspondence analysis (DCA), Mantel test, and stepwise multiple linear regressions were used in this study. Canopy communities were significantly different in basal area (BA), density, and species richness but not Shannon diversity (H’). Soil variables differed in terms of soil pH and TN across the three canopy types; however, the light levels did not differ significantly. MRPP revealed significant differences in species composition of understory among the three forest types. DCA ordination separated sample plots into three groups corresponding to the three forest types. Multiple stepwise regressions showed that soil pH combined with SOM, Overstory-H′ and TTot could explain 99% of the variation in understory species richness, while Overstory-H' with Overstory-S, Density, and SOM could explain 92% of the variation in understory diversity. The results indicated that canopy structure together with soil nutrient were the best predictors of understory vegetation in the study area.