Abstract: We studied the effects of exogenous spermidine(Spd) on free polyamines content and polyamine biosynthesis gene expression in cucumber(Cucumis sativus L.cv.Jinchun No.2) seedlings under salt stress with solution culture.Plant height,stem diameter,and fresh and dry weight were significantly reduced by treatment with 75 mmol/L NaCl,while the inhibition of seedling growth was alleviated by spraying 1 mmol/L Spd.Free polyamine content was significantly increased in leaves when exposed to salt stress,and exogenous Spd application to leaves further promoted free Spd and spermine(Spm),but reduced free putrescine(Put).Expression analysis of polyamine synthase genes showed that salt stress up-regulated adc,odc,samdc and spds gene expression,and exogenous Spd application further up-regulated samdc gene expression and down-regulated adc,odc,spds gene expression.The results revealed that exogenous Spd regulated polyamine metabolism in cucumber seedlings under salt stress,inhibited free Put accumulation by reducing adc,odc gene expression,increased free Spd and Spm accumulation by up-regulated samdc gene expression,and alleviated the inhibition of plant growth under salt stress.