A controlled hydroponics experiment in the tall plant growth rooms (AGC-DOO3N) was conducted to investigate the effects of elevated CO₂ concentration(at two controlled levels:380 and 760 μmol·mol^-1) on the agronomic traits,dry matter accumulation of maize and soybean under three cropping patterns and two phosphorus levels(phosphorus deficiency and normal phosphorus supply).The three cropping patterns are maize monoculture,soybean monoculture and maize-soybean intercropping.The results showed that:(1)Elevated CO₂ could significantly increase plant height,stem diameter,leaf area and the root dry weight, above-ground dry matters weight as well as gross dry matters weight of maize and soybean under different cropping patterns.(2)Compared to the phosphorus deficiency group,this kind of positive effect on every trait was better promoted in the normal phosphorus supply group.(3)Cropping patterns did not affect the growth of soybean under both CO₂ concentrations in this study.Intercropped maize had,however,significant growth advantage over monocultured maize,and this advantage could be enhanced under higher phosphorus supply and CO₂ concentration.