Light is a critical factor influencing the survival and growth of saplings. Rhododendron schlippenbachii, an ornamental species endemic to Changbai Mountain in China, has been classified as Near Threatened (NT) on the IUCN Red List of Threatened Species. However, the optimal light intensity for its growth remains unclear. To determine this, we conducted a comprehensive investigation into the adaptation mechanisms of morphological growth, diurnal variations in photosynthesis, chlorophyll fluorescence, physiological indicators, and stomatal ultrastructure of Rhododendron schlippenbachii saplings under four light intensities (full light: CK; 70% light: L1; 50% light: L2; 30% light: L3) over a continuous 90-day shading period. The results demonstrated that under L3, the leaf morphological growth of Rhododendron schlippenbachii saplings was significantly greater than that of other treatments. As shading duration increased, saplings in the full-light treatment group experienced complete mortality. The highest values for Fv/Fm, Fv/F0, and NPQ, as well as the peak chlorophyll content, were observed in saplings subjected to 70% shading. Conversely, the lowest concentrations of proline (PRO), malondialdehyde (MDA), and soluble protein (SP) were also recorded under this condition. Activities of catalase (CAT), superoxide dismutase (SOD), and peroxidase (POD) did not exhibit significant differences across various shading treatments. It was observed that the protective effect was significantly enhanced by antioxidant enzymes under 70% shade conditions; shading reduced osmoregulatory substances while increasing chlorophyll content. The stomatal length, width, conductivity, and net photosynthesis rate of the 30% light treatment surpassed those under full-light conditions after 60 days of shading. Consequently, this study demonstrated that a 70% shade treatment represents the optimal level of light radiation for the growth of Rhododendron schlippenbachii saplings.