The sensitive response to light environment makes the common light heterogeneity in forests to be a key factor affecting plant self-renewal. However, the underground root structure of plants lacks in-depth research due to its difficulty in measuring light. In order to further understand the root response strategy of Casuarina equisetifolia under different light intensities, one-year-old C. equisetifolia seedlings were used as experimental materials, and four light treatments were set up to simulate the light environment outside the forest (CK), in the forest edge (L1), in the forest gap (L2), and under the forest (L3). The growth, root morphology, anatomical structure of roots and carbon and nitrogen contents of the seedlings were measured and analyzed. The results showed that: (1) Under L1, the seedlings took to maintain height and reduce lateral growth to ensure normal accumulation of biomass. With decreasing of light intensity, the plant height, ground diameter, leaf biomass and aboveground part of the biomass gradually decreased. (2) In terms of root phenotype, the seedlings showed a tendency to inhibit longitudinal elongation and promote lateral root growth with the increasing light limitation, in which total root length, mean root diameter and root volume reached significant differences. In terms of radial structure, the development of fine roots decreased with the decreasing light. The coarse root growth was promoted in appropriate shading(L1), and the root length, root surface area, and root volume decreased significantly in excessive shading(L3), except for a rise in the number of root tips compared with CK. (3) The anatomical changes of grade 1-3 fine roots were large. Compared with CK, the cortical cell area of fine roots in grade 1 increased significantly, while the root radius, vascular column structure and epidermal thickness decreased apparently. The root radius, cortical cell area and epidermal thickness of grade 2 fine roots decreased markedly, while the vascular column structure decreased obviously only at L2 and L3. The root radius, cortical cell area and vascular column area of grade 3 fine roots increased significantly. Although the vascular column structure decreased at L1, both the area of the vascular column and the percentage of the middle column increased significantly with the continuous decrease of light. (4) There was no significant differene between CK and L1 in terms of carbon and nitrogen content. At L2, TC decreased significantly and TN increased significantly, both of which reached the maximum at L3. C:N decreased gradually with decrease of light intensity. In summary, when light is restricted, the biomass and carbon allocation of C. equisetifolia stabilized growth in the rhizome portion of the plant, adopting a contraction growth strategy of "weak uptake and strong storage". However, when the light limitation intensified, the imbalance of photosynthesis and respiration led to the imbalance of plant nutrient inputs to fine roots, and eventually caused the death of trees. The results provide a theoretical reference for the regeneration of understory vegetation.