Abstract: A pot experiment was conducted to determine the effects of drought stress on the photosynthetic characteristics, antioxidant properties and osmotic adjustment substances of the desert shrub Hedysarum mongolicum. The experiment used four different water treatments of 70%, 50%, 30% and 15% field moisture capacity, representing serious (SED), moderate (MOD) and mild (MID) stresses and the control (CK), respectively. The photosynthetic characteristics (except intercellular CO₂ concentration) under MOD and SED were significantly lower than those under CK. Compared to CK, the net photosynthetic rate and PSⅡ potential activity under MID increased 20.7% and 5.7% respectively. The malondialdehyde content, relative plasma permeability, and the activities of superoxide dismutase, catalase and peroxidase were significantly higher under the MOD and SED treatments than CK, while there was no significant difference between MID and CK. The contents of proline, soluble sugar and K⁺ in leaves showed significant increase trends with the degree of drought stress. Compared to CK, Na⁺ contents in leaves, roots and stems showed significant decline trends. These results show that in response to stress H. mongolocum increases the activities of SOD (superoxide dismutase), CAT (catalase) and POD (peroxidase), increases relative plasma permeability in leaves, improves the contents of proline and soluble sugar, and enhances the selective absorption of K⁺, thereby reducing the damage of drought to the plant. Our findings provide the theoretical basis for understanding H. mongolocum adaptation to drought stress. The study also provides references for the protection and utilization of this plant and for vegetation restoration in arid and semiarid areas.