Carbon dioxide (CO2) is the most important anthropogenic greenhouse gas, the global atmospheric CO2 concentration (\[CO2\]) has increased from a pre-industrial value of about 280 to 379 mol/mol in 2005, which was directly resulted from human activities. With the ongoing rise in world population and economic activity, atmospheric \[CO2\] is expected to double its current level at the end of 21st century and will reach to 730-1020 μmol/mol in 2100. The doubled atmospheric \[CO2\] and a rise of other greenhouse gases could lead to available soil water shortage in many areas of the world. Thus it is important to consider both elevated CO2 concentrations and different water conditions in order to assess the possible effects of climate change on plants. This study aimed to explore the effect and its mechanism of doubled CO2 concentration on oxidative damage of cucumber seedlings under drought stresses, and then provide theoretical basis and technical parameters for the high quality and efficient cultivation of cucumber. Osmolates, lipid peroxidation and antioxidant system of hydroponic cucumber seedlings (Cucumis sativus L. var. Jinyou No.1) were investigated under atmospheric and doubled CO2 concentration (760±20 μmol/mol), companied with drought stresses simulated by PEG 6000. Split-plot design was employed in the experiment, the main treatment was CO2 concentration including two levels (Ambient \[CO2\] ≈380 μmol/mol and Doubled \[CO2\]=760±20 μmol/mol), the subplot treatment was drought stress treatment including three levels (control was Yamazaki cucumber nutrient solution indicated as C; moderate drought stress was Yamazaki cucumber nutrient solution + 5% PEG 6000, water potential ψw=-0.05MPa, indicated as M; severe drought stress was Yamazaki cucumber nutrient solution + 10% PEG 6000, water potential ψw = -0.15MPa, indicated as S). The results showed that: (1) drought stresses induced the accumulation of reactive oxygen species, subsequently resulted in the increase of MDA content and permeability of cell membrane as well as osmoticum (proline, soluble protein and soluble carbohydrate) content. Meanwhile, the antioxidative enzymes (SOD, POD, CAT, APX and GR) activities, AsA and GSH contents increased significantly with the aggravation of drought stress; (2) under the condition of severe drought stress, doubled \[CO2\] remarkably increased proline content; under the conditions of both moderate and severe drought stress, doubled \[CO2\] significantly enhanced soluble protein and soluble carbohydrate content; (3) doubled \[CO2\] markedly reduced permeability of cell membrane by 17.1% under severe drought stress and reduced MDA content significantly by 94% and 16.4% under moderate and severe drought stress, respectively; (4) doubled \[CO2\] increased activities of SOD, POD, APX and GR under moderate drought stress, while increased the all antioxidative enzymes activities significantly under severe drought stress by 17.6%, 15.5%, 10.6%, 11.4% and 8.8%, respectively; (5) doubled \[CO2\] increased AsA content significantly by 37.7% under severe drought stress, and there was no significant difference for GSH content between ambient and doubled \[CO2\]. In conclusion, not only did doubled CO2 concentration promote osmolates accumulate, but also enhanced the activities of antioxidative enzymes and increased the content of AsA and GSH in cucumber seedlings under drought stresses condition, and then reduced the accumulation of ROS, decreased lipid peroxidation and cytomembrane permeability as well as MDA content. Based on the above results, we speculate that increasing CO2 concentration can enhance the resistance of cucumber seedlings to drought stress and mitigate the negative effects of drought stress to some extent.