Abstract: The experiment was carried out to study the effects of 1mM exogenous spermidine( Spd) on radish seedling growth, the photosynthetic pigments content in leave, soluble protein content, MDA, Activity of Antioxidant Enzymes. The radish seedling was exposed to high temperature (40℃/30℃) day alternates with night for one to five days with solution culture. The results show that with the increasing duration of high temperature stress seedling growth was significantly inhibited. The amount of photosynthetic pigments content in leaves and soluble protein content of seedlings decreased, and the amount of MDA increased. Antioxidant enzyme activities including POD, CAT, SOD and PAX in seedling increased first, and decreased finally. Exogenous Spd spray could promote the seedling growth. Suitable Spd concentration enhanced the activities of CAT, POD, SOD, APX, photosynthetic pigments, and soluble protein content; while the content of MDA decreased. The Spd concentration can also reduce the production O_-2 rate and H₂O₂ content, maintain the stability and integrity of cell membrane structure. Finally enhanced high temperature tolerance of radish Seedlings.