Abstract: To investigate the photosynthetic and physiological adaptation mechanisms of Prunus mira Koehne seedlings under drought stress, this study established four soil relative water content gradients (85%, 65%, 45%, and 25%) and comprehensively analyzed their growth, photosynthetic characteristics, antioxidant systems, and osmotic regulation dynamics through potted water-controlled experiments. Results indicate: Under mild drought conditions (65% soil relative water content), growth and photosynthetic capacity of Prunus mira seedlings showed no significant inhibition. Net photosynthetic rate and stomatal conductance remained at relatively high levels, while antioxidant enzyme activity and osmotic regulatory substance content exhibited minimal changes, demonstrating good physiological stability. Under moderate drought (corresponding to 45% soil water content), seedlings significantly enhanced drought resistance by optimizing stomatal conductance and carbon allocation, exhibiting compensatory growth characteristics; Under severe drought (corresponding to 25% soil relative water content), phototrophic walnut seedlings experienced accelerated membrane lipid peroxidation, collapse of the photosynthetic system, and a significant decline in antioxidant enzyme activity. Concurrently, excessive accumulation of proline and soluble sugars triggered metabolic imbalance. The “rapid osmotic regulation-short-term tolerance” strategy adopted by photogalactica seedlings aligns with the frequent short-term droughts in their native habitat but is prone to causing irreversible damage under persistent extreme drought. This study reveals that light walnut seedlings exhibit distinct phased responses to drought stress. Mild to moderate drought induces effective adaptive regulatory mechanisms, whereas prolonged severe drought readily induces physiological decline and irreversible damage.