Abstract: In order to understand the effects of rhizosphere CO₂ concentration on root growth and contents of four hormones in xylem sap, areoponic cultivation experiments were conducted during anthesis and fruiting periods on root system of metted muskmelon with different CO₂ concentrations. The results showed that in treatments of CO₂ concentrations of 2500 and 5000 μL/L, root growth was inhibited and root activities were increased and then decreased compared with a treatment of CO₂ concentration of 350 μL/L. At the same time, xylem sap volume,conductance, pH value and promoting hormone contents such as Indole acetic acid, Zeatin, Gibberellin A3, were decreased, but Abscisic acid content increased. The results show that the growth and hormone synthesis of root can be influenced when the rhizosphere CO₂ concentration is greater than 2500 μL/L. It may be the important reason why areoponic cultivation advances the yield of plant compared with soil cultivation and substrate soilless cultivation.