Abstract: In order to make clear the physiological and biochemical mechanisms of exogenous salicylic acid (SA) on salt resistant capability of alfalfa, Medicago sativa cv. Gannong No.3 was used as material to study the effect of exogenous SA on alfalfa seedling growth, organic osmotic regulation substance contents and anti-oxidization system on condition of 150 mmol/L NaCl stress using foliar application. The result indicated that 150 mmol/L NaCl salt stress had conspicuous inhibitory effect on alfalfa seedling growth, while significant increase in shoot heigth, root length, fresh weight and plant chlorophyll content were found when 0.25 mmol/L exogenous SA had been applicated, together with significant decrease in free proline and malondialdehyde (MDA) contents in leaves and rootsand increase in soluble protein; superoxide dismutase (SOD), guaiacol peroxidase (GPX) and ascorbate peroxidase (APX) activities in leaves were found significantly increased, and no change was found in catalase (CAT), ascorbic acid (AsA) and reduced glutathione (GSH); for the root, GPX activity, AsA and GSH contents were found increased, and no change was found in SOD, CAT and APX activities, indicating that exogenous SA could regulate organic osmotic regulation substances and protect the anti-oxidization system in alfalfa seedling, and alleviate the injure of salt stress on alfalfa plants.