Abstract: This paper studied the effects of 2，4-epibrassinolide (EBR) on the photosynthesis and antioxidant system of cucumber seedling leaves under NaCl stress. As compared with the control, NaCl stress increased the leaf superoxide anion production rate, hydrogen peroxide content, malondialdehyde content, and cell membrane permeability while decreased the leaf net photosynthetic rate, stomatal conductance, transpiration rate, and intercellular CO₂ concentration significantly, and inhibited the seedlings growth significantly. Applying EBR could increase the activities of leaf superoxide dismutase, peroxidase, and catalase, decrease the leaf superoxide anion production rate, hydrogen peroxide content, malondialdehyde content, and cell membrane permeability, make the leaves keep a higher photosynthetic rate, and thus, promote the seedlings growth, being able to effectively alleviate the damage of NaCl stress.