Abstract: In order to explore the correlation between the composition and diversity pattern of understory plants and environmental factors, and to reveal the response of understory plant to environmental factors, 47 sample plots were set up in the Pearl River Delta to investigate the understory plants. We used Spearman nonparametric correlation analysis to assess the relationship between environmental factors such as elevation, slope, aspect, slope position, canopy openness, understory radiation and diversity indices of understory plants. Random forest analysis method was used to predict the importance of environmental factors to understory diversity. Fourth-corner analysis was used to examine the significance of the relationship between functional group and environmental factors, and to reveal the response of understory functional groups to environmental change. The results showed that there were significant differences in the composition and diversity pattern of understory plants among different forest types (P<0.01). The composition and diversity pattern of understory plants are affected by many environmental factors, and the importance of each factor is different. Random forest analysis showed that the number of stems and Simpson index of undergrowth plants are mainly affected by forest types. Aspect, slope and forest types play an important role in the number of species of undergrowth plants. The transmitted totals radiation has a determine effect on the evenness of undergrowth plants. And elevation and forest types play a dominant role in Shannon-Wiener index of undergrowth plants. For the same environmental factor, there were differences in the response and adaptation strategies of different understory functional groups. Habitat heterogeneity and plant biological characteristics are the dominant factors affecting understory plant diversity.