Understory plant communities are important components of forest ecosystems, and play a crucial role in the regulation of community structures and functions. The present study focused on the key controlling factors of the species composition of understory plants in a Castanopsis hystrix plantation chronosequence by combining field investigation and laboratory analysis. The objectives were to determine the relationship between understory plant species composition and environmental factors, and their key controlling factors. The results showed significant variations in the plant species composition of shrub and herb layers among stand age classes in the Castanopsis hystrix forest (P<0.05). Redundancy analysis suggested that basal area was the most important factor determining the shrub layer plant distribution, and basal area, slope, and elevation were the most important factors determining herb layer plant species compositions. Variation partitioning analysis revealed that the two categories of factors, i.e. biotic and abiotic factors, explained 63% and 47% of the variation in the shrub and herb layer species composition, respectively. The shared effect of the microenvironment and plant factors (i.e. slope, elevation and basal area) was the most important combined factor determining understory plant distribution. However, the shared effects of other combination of factors were insignificant. The effect of microenvironmental factors was more important than the effect of soil or plants in determining understory plant distribution in the present study sites.