Abstract: The effects of 24epibrassinolide (EBR) on antioxidant system and photosynthesis of cucumber seedlings were studied under suboptimal temperature, light and salt environment. Compared with the control, the leaf H₂O₂ content, lipid peroxidation and cell membrane permeability significantly increased, and the leaf net photosynthetic rate (P_n), stomatal conductance (g_s), intercellular CO₂ concentration (C_i), transpiration rate (T_r) and dry matter accumulation decreased by 39.3%, 40.0%, 21.2%, 47.2% and 35.9% in the suboptimal environment, respectively. Applying EBR could further improve the antioxidant enzyme activity, reduce the H₂O₂ content and membrane permeability of seedlings, alleviate the drop range of P_n, g_s and T_r, improve the growth of seedlings and increase the dry matter accumulation by 25.9%. Therefore, EBR treatment could keep higher photosynthetic performance to effectively promote cucumber seedlings growth through adjusting the protective enzyme activity and reducing membrane lipid peroxide level under suboptimal temperature, light and salt environment.