Abstract: Under simulated environment of 35℃ and 700μmol/mol CO₂，the activities of antioxidant enzymes and changes of lipid peroxidation in Larix principis-rupprechtii Mayr seedlings were investigated. The results show that activities of superoxide dismatase (SOD), peroxidase (POD) and catelase (CAT) are decreased under the stress of high temperature, while increased under doubled CO₂ and the complex stress of high temperature and doubled CO₂. The content of malondialdehyde (MDA) raises under high temperature, but reduces under doubled CO₂, as well as the complex stress. The results indicate that doubled CO₂ could effectively elevate the defensive capacity of Larix principis-rupprechtii Mayr seedlings against the stress of high temperature by improving antioxidant enzyme activity.