Abstract: The effects of Na⁺ on the photosynthetic characteristics, protective enzyme activities and contents of organic osmotic adjustment substances in the seedlings of Zygophyllum xanthoxylum under different osmotic stresses (-0.5, -1.0 and -1.5 MPa) were measured in order to explore the significance of Na⁺ in the drought resistance of Z. xanthoxylum. Under different osmotic stresses, 50 mmol/L NaCl significantly increased the net photosynthetic rate and superoxide dismutase (SOD) and catalase (CAT) activities in leaves, but the soluble sugar and free proline contents decreased. The contents of free proline decreased by 66%, 55%, 31% (in -0.5, -1.0 and -1.5 MPa respectively) and contents of total sugar decreased by 41%, 33%, 30%. Since our previous experiments have shown that Na⁺ concentration in Z. xanthoxylum increased considerably under 50 mmol/L NaCl, it indicated that the reason why Na⁺ can improve the drought resistance of Z. xanthoxylum is not the promoting effect of Na⁺ on the accumulation of the organic osmotic adjustment substance, but the accumulation of Na⁺ as an osmotic adjustment substance in the plant.