A hydroponic experiment was carried out with two different tomato cultivars in salinity-alkalinity tolerance to explore the effects of exogenous spermidine (Spd) soaking seeds on relative growth rates of plant height and stem diameter,contents of leaf water and root water,malondialdehyde (MDA),proline (Pro),chlorophyll,activities of superoxide dismutase (SOD) and peroxidase (POD).There are four processing:normal nutrient solution cultivation (CK),100 mmol/L salinity-alkalinity stress,100 mmol/L salinity-alkalinity stress+pre-soaked seeds with 0.1 mmol/L spermidine (Spd),100 mmol/L salinity-alkalinity stress+pre-soaked seeds with 0.25 mmol/L Spd.The results showed that:(1)The chlorophyll content,relative growth rate of plant height and stem diameter,water content in leaf and root reduced;the contents of MDA and Pro increased;the activities of SOD and POD enhanced under salinity-alkalinity stress.(2)Pre-soaked seeds with exogenous Spd could alleviate damage of salinity-alkalinity stress on tomato seedlings via increasing relative growth rates of plant height and stem diameter,water contents in leaf and root,reducing content of MDA,increasing contents of Pro and chlorophyll,enhanceing the activities of SOD and POD in tomato seedlings under stress condition.(3)Under 100 mmol/L salinity-alkalinity stress,0.25 mmol/L Spd has conspicuous impact on alleviating the harm of tomato seedlings,which was more significant for sensitive salinity-alkalinity ‘Zhongza No.9’ seedlings.In summary,the growth of tomato seedlings by a certain degree of inhibition under salinityalkalinity stress,but exogenous Spd could keep tomato seedlings maintain the normal physiological metabolism and improve the seedling resistance to salinity-alkalinity stress ability by adjusting osmotic adjustment material contents and increasing antioxidant enzyme activities to alleviate damage caused by salinity-alkalinity stress.0.25 mmol/L exogenous Spd could enhanced tolerance of tomato plants to salinity-alkalinity stress and this relieve effects on ‘Zhongza No.9’ seedlings is obvious.