The relationship between light conditions and saplings' functional traits is of great ecological significance to the regeneration and succession of natural forests. In order to reveal the relationship between canopy structure and understory light, as well as the response of undergrowth saplings' functional traits to the light, we investigated three natural evergreen broad-leaved forests of different ages (young forest, middle-aged forest, old-growth forest) in the Nanling Mountains of Guangdong Provinces. The results showed that: (1) The leaf area index of the middle-aged forest was significantly higher than that of the young and old-growth forests (P<0.05). Canopy openness and light transmittance gradually decreased with increasing forest age. Light transmittance, R/FR (red/far-red ratio of light) and Bw/Rw (broadband-blue/broadband-red ratio of light) of evergreen broad-leaved forests significantly differed among forest sets (P<0.001). (2) A significant correlation existed between light transmittance and light quality in natural evergreen broad-leaved forests (P<0.001). As light transmittance increased, R/FR increased while Bw/Rw decreased. (3) The functional traits of undergrowth saplings differed significantly among the different light environments (P<0.05). In the old-growth forest, leaves of undergrowth saplings had higher nitrogen content and were less overlapped than that of the young forest. (4) In the experimental plots, the changes of R/FR and Bw/Rw showed no significant effect on the height-diameter ratio and photosynthesis of understory plants, while the light intensity greatly influenced the maximum net photosynthetic rate of the same species. In general, structural differences in canopies and the light environment across the three forests can explain the variation in the functional traits of saplings to some extent, which may facilitate an understanding of the influence of light on understory sapling regeneration, and provide guidance for natural vegetation restoration and forest management.