Taking “Xinghai 12” tomato as the research object,four different concentrations of CO₂ and four different concentrations of selenium were set up in this experiment to study the effect of CO₂ fertilization on selenium accumulation in tomato. The four CO₂ concentrations and four selenium levels were atmosphericconcentration(CK),(600±25)(T1),(800±25) (T2),(1000±25)(T3)μmol·mol-1 CO₂,and0(L0),2(L1),4(L2),6(L3)mg·L-1Se,respectively. Under the same CO₂ condition,the contents of total selenium and organic selenium in tomato fruit increased with the increase of seleniumconcentration,and when the concentration of selenium is thesame,with the increase of CO₂ concentration,both total selenium and organic selenium in tomato fruit increasedsignificantly,the effect of 800μmol·mol-1 treatment was the best. This experiment was conducted not only to study the effects of CO₂ on selenium accumulation and transformation intomato,the main nutrient contents of mature fruits under different combinations of CO₂ and sodium selenite were also compared. The results showed that all the application of selenium and CO₂ alone and the combination of selenium and CO₂ improved the nutritional quality of tomato. The content of vitaminC,lycopene and soluble sugar in fruit increased significantly when the concentration of T2L3(CO₂ was 800 μmol·mol-1 and the concentration of selenium was 6 mg·kg-1). When the concentration ofT3L2(CO₂ was 1 000 μmol·mol-1 and the concentration of selenium was 4 mg·kg-1),the ratio of sugar to acid is higher.