The phenotypic plasticity of plants in varied light environments affects their distribution, growth, and regeneration in the understory environment. To investigate the phenotypic plasticity and adaptation mechanisms of Illicium lanceolatum in different light conditions, five light conditions (100%, 52%, 33%, 15%, and 6% relative light intensity) were simulated in the shading experiment. The changes in leaf morphology, physiology, anatomical structure, root morphology, and biomass allocation of I. lanceolatum in different light conditions were studied. The results showed that there was no significant difference in leaf biomass between the five light treatments, and the leaf area and specific leaf area increased significantly with the decrease of light intensity. The shading treatments enhanced the contents of chlorophyll a, chlorophyll b, and carotenoid. However, the chlorophyll a/b ratio decreased with the decrease of light intensity. Shading reduced the contents of non-structural carbohydrates (starch and soluble sugar) and soluble protein, and increased the leaves' nitrogen and phosphorus contents, but the nitrogen/phosphorus ratio of leaves was less affected by light gradients. The nitrate content of the leaves in 52% and 33% relative light treatments was the lowest, whereas the nitrate content was higher in the 100% and 6% relative light treatments. Root biomass, fine root and root length, root surface area, specific root length, and specific root surface area were not significantly different among the five light treatments. The content of root nitrogen was significantly reduced in the low light environments (15% and 6% relative light intensities). As light intensity decreased, I. lanceolatum adopted a conservative survival strategy, which did not invest in leaf biomass, but allocated more biomass to the branches and trunks to store energy. As a whole, I. lanceolatum had a wide light ecological amplitude and could grow normally in 6%-100% light intensities. Shading was beneficial for the accumulation of aboveground biomass and total biomass for I. lanceolatum, and the optimal growth condition was 52% relative light intensity. The plasticity of the root traits and whole structure was relatively lower in the different light environments, and the plasticity of the leaf physiological traits played a key role for I. lanceolatum to adapt to varied light environments.