Abstract: Taking melon ‘Yumeiren’ as the test material, and the aeroponics culture CO₂ automatic system, the changes of root growth indexes and auxin（IAA）,cytokinin（CTK）,gibberellin（GA） and abscisic acid（ABA） content in the roots of melon under six different rhizosphere CO₂ concentrations（0.2%,0.4%,0.6%,0.8%,1.0% and 1.2%,0.2% of the conventional root-zone CO₂ concentration were used as the control） were studied, in order to provide reference for in-depth study of the internal mechanism of plant growth under root-zone high CO₂ concentration.The results showed that with the increase of CO₂ concentration and the prolongation of treatment time, the growth indexes and the content of growth-promoting hormones（IAA,CTK and GA） of melon showed a trend of first increasing and then decreasing.Before 6 days of CO₂ treatment, the root length, root surface area, total number of root tips, mean diameter of root, root activity, IAA,CTK and GA content were significantly higher than the control, and the higher the concentration, the more the increase.After 9 days of treatment, root growth was inhibited to varying degrees.The content of IAA,CTK and GA decreased with the increase of CO₂ concentration, while the content of ABA gradually increased.The concentration of root-zone CO₂ was different, and the time for the hormone to reach the peak was different.The higher the concentration of CO₂,the earlier the time for the hormone content to reach the peak.In conclusion, the high root-zone CO₂ concentration of melon could promote the growth and the synthesis of growth-promoting hormones（IAA,CTK and GA） of melon seedling roots in a short period of time（＜6 days）.With the extension of the enrichment time, the content of IAA,CTK and GA decreased, and the growth of root was inhibited.