We investigated the effects of CO₂ enrichment on photosynthetic characteristics and metabolism of reactive oxygen species in the leaves of cucumber (Cucumis sativus L. ‘Jinyou No.35’) seedlings under salt stress. In the split-plot design used, the main treatment consisted of two CO₂ concentration levels[ambient[CO₂] ≈400 μmol/mol and enriched[CO₂]=(800 ±40) μmol/mol] and the subplot had two levels of salinity treatment (0 and 80 mmol/L NaCl). The results showed that after 7 days of experimental treatment, salt stress caused a significant reduction in growth rate (i.e., increase in plant height, stem thickness, leaf area, and the growth of both shoot and root), ribulose-1,5-bisphosphate carboxylase (RuBPCase) activity, net photosynthetic rate (P_n) of the leaves, stomatal conductance (G_s), and transpiration rate (T_r) of cucumber seedlings. Salt stress decreased the chlorophyll[chlorophyll a, chlorophyll b, carotenoids, and chlorophyll (a+b)] content, electron transport rate (ETR), quantum of PSⅡ (φ_PSⅡ), and photochemical quenching (qP) but markedly increased non-photochemical quenching (NPQ); there were no significant differences in chlorophyll a/b and maximum quantum yield of PSⅡ (F_v/F_m) between control and salt stress treatments. Moreover, the portion of excessive energy in the photosystem Ⅱ (PSⅡ) reaction center was enhanced by salt stress that increased the rate of production of the superoxide anion radical (O₂^·-) and hydrogen peroxide (H₂O₂) content in the leaves of cucumber seedlings. Subsequently, this resulted in an increase in malondialdehyde (MDA) content, permeability of cell membranes, and osmoticum (proline) content. Changes in antioxidative enzyme activities of cucumber seedling leaves differed between the salt stress and elevated[CO₂] treatments. Superoxide dismutase (SOD) and catalase (CAT) activities increased while peroxidase (POD) activity decreased in cucumber seedling leaves treated with salt stress. However, the activity of antioxidative enzymes (such as SOD, POD, and CAT) and proline content were enhanced by elevated[CO₂]. Therefore, we can speculate that reactive oxygen species (ROS) quenching was limited under salt stress, leading to their accumulation and serious lipid peroxidation. However, elevated[CO₂] reduced the content of MDA, H₂O₂, and the rate of O₂^·- production in cucumber seedling leaves under salt stress. ROS was eliminated by antioxidative enzymes under elevated[CO₂], therefore, lipid peroxidation damage was less than that caused by salt stress. Elevated[CO₂] increased the plant height, stem thickness, leaf area, and shoot fresh weight of cucumber seedlings significantly under salt stress. Additionally, elevated[CO₂] decreased the content of chlorophyll[chlorophyll a, chlorophyll b, carotenoid, and chlorophyll (a+b)], stomatal conductance, and transpiration rate but markedly increased P_n, ETR, φ_PSⅡ, and RuBPCase activity in leaves of cucumber seedlings under salt stress. In conclusion, elevated[CO₂] alleviated the negative effects of salt stress and enhanced the resistance of cucumber plant growth to salt stress through enhanced P_n, cellular membrane stability, proline content, and antioxidative enzyme activities while transpiration rate, MDA content, and accumulation of ROS were reduced.