To further elucidate the relationship of spermidine and salt tolerance of halophyte Nitraria sibirica Pall.,under different levels of salinity in hydroponics condition,we studied the effect of spermidine on malondialdehyde (MDA),superoxide anion (O^-·₂) and the antioxidant enzyme system in leaves of N.sibirica seedlings.The result showed that in treatment groups (100 mmol·L^-1 NaCl and 200 mmol·L ^-1 NaCl),the activities of superoxide dismutase (SOD),peroxidase (POD) and catalase (CAT) in leaves of N.sibirica seedlings and the activity of root system were significantly increased,while the content of the MDA and O^-·₂ in leaves were reduced in 5 d after foliage spray of spermidine (0.1 mmol·L^-1).In treatment groups (0,50 and 300 mmol·L ^-1 NaCl),there was no obvious effect on leaves of N.sibirica seedlings after treatment of exogenous spermidine.The results showed that under 100 mmol·L^-1 to 200 mmol·L ^-1 NaCl stress,exogenous spermidine can effectively reduce the injury to N.sibirica and enhance the adaptability in salt environment.