The effects of doubled CO₂ concentration on non-structural carbohydrate metabolism of cucumber (Cucumis sativus L. cv. ‘Jinyou No.1’) seedlings under drought stress were investigated. Split plot design was deployed, with two levels of CO₂ concentrations (ambient CO₂ concentration, 380 μmol·mol^-1, and doubled CO₂ concentration, 760±20 μmol·mol^-1) in the main plots, and three levels of water treatments (control, moderate drought stress, and severe drought stress) simulated by PEG 6000 in the splitplots. The results showed that nonstructural carbohydrates of cucumber leaves, including glucose, fructose, sucrose, and stachyose, increased with the doubling of CO₂ concentration, which resulted in the decreased osmotic potential, improving the drought stress in cucumber seedlings. During the drought stress, sucrose synthase, soluble acid invertase and alkaline invertase started with an increase, and followed with a decline in the leaves. In the root system, however, soluble acid invertase and alkaline invertase increased gradually throughout the whole process, whereas sucrose phosphate synthase firstly increased and then decreased. The treatment of doubled CO₂ enhanced the activity of sucrose synthase, but decreased the activity of sucrose phosphate synthase. The synergistic action of the two enzymes and invertase accelerated the decomposition of sucrose and inhibited the synthesis of sucrose, leading to the accumulation of hexose, which lowered the cellular osmotic potential and enhanced the water uptake capacity. In conclusion, doubled CO₂ concentration could alleviate the adverse effects of drought stress and improve the drought tolerance of cucumber seedlings. Such mitigating effect on cucumber was more significant under severe drought stress.