Abstract: The study focuses on 5-year-old Cunninghamia lanceolata saplings at 3 afforestation densities(833, 2500,6667 trees/hm²), analyzing the forest light environment, changes in understory vegetation diversity, and root competition to examine the effects of afforestation density on understory vegetation composition and root competition. The findings revealed that increased afforestation density significantly affected the light environment within the forest, with total radiation under the canopy at densities of 833, 2500, and 6667 trees/hm² being 77.82, 31.94, and 14.03 μmol/(m²·s), respectively, and total illuminance in the understory being 5893.13, 1130.7, and 761.5 Lx, respectively. The Shannon−Wiener index and Pielou's evenness index of the herb layer showed an initial increase followed by a decrease with increasing density; the Simpson's dominance index and Pielou's evenness index of the shrub layer decreased with increasing afforestation density. As density increased, the biomass of C. lanceolata fine roots and the C. lanceolata root competition index both gradually increased, while the biomass of understory vegetation and its root competition index gradually decreased. Light environment indicators were significantly positively correlated with the Pielou's evenness index of the shrub layer and the Simpson's dominance index of the herb layer, significantly negatively correlated with the C. lanceolata root competition index, and showed a highly significant positive correlation with the root competition of understory vegetation. From this, changes in afforestation density significantly affect the forest's light environment, with light-related indicators decreasing as afforestation density increases, leading to a reduction in the biomass of understory vegetation in C. lanceolata saplings and affecting the diversity changes of understory vegetation, as well as enhancing the root competition ability of C. lanceolata. Considering the growth condition of C. lanceolata, an afforestation density of 2500 trees/hm² is deemed more suitable for constructing high-quality C. lanceolata plantations.