The objective of this study was to investigate the effects of exogenous application of spermidine on the growth, active oxygen scavenging system and bound polyamine contents in chloroplasts of cucumber under salt stress. The results showed that under salt stress, exogenous spermidine increased TGase activity and bound putrescine, spermidine, spermine and total polyamine contents in chloroplasts. The activities of SOD, APX and GR, the contents of AsA, Car and GSH, and the ratios of GSH/GSSG in chloroplasts were increased, while the ratios of DAsA/AsA and the contents of H2O2 and MDA decreased. Furthermore, exogenous spermidine increased net photosynthetic rates, as well as alleviated the salt-induced plant growth inhibition. These results suggest that exogenous application of spermidine could alleviate salt-induced membrane injury of chloroplast by increasing the active oxygen scavenging ability and the bound polyamine contents in chloroplast.