Cucumber was hydroponically cultured to study the effects of elevated CO2 concentration on the growth, photosynthetic characteristics and mineral elements of cucumber seedlings under salt stress. The results indicated that the biomass, net photosynthetic rates, stomatal conductance and transpiration of cucumber seedling grown in 80 mmol/L NaCl decreased compared to the control, while intercellular CO2 concentration increased. However, under NaCl stress, elevated CO2 concentration enhanced the biomass, net photosynthetic rates, stomatal conductance and transpiration of cucumber seedling compared to that at ambient CO2 concentration. The results indicated that the unfavorable effects of salt stress on photosynthetic functions could be alleviated by high CO2 concentration. 80 mmol/L NaCl reduced total N and K+ content, and enhanced Na+ content of cucumber seedling. However, under salt stress, elevated CO2 concentration increased total N and K+ content, and decreased Na+ content of cucumber seedling, indicating that elevated CO2 concentration alleviated salt stress on cucumber seedling and enhanced biomass of cucumber seedling.