Abstract: In order to investigate the impact of salt stress on growth and photosynthetic physiological characteristics in alfalfa (Medicago sativa) provided scientific foundation for enhancement of plant salt tolerance , increase of biomass and breeding new salt-tolerant plant.The two-year Gongnong No.1 as experimental material to study physiological and biochemical responses under different concentration of NaCl in fields. The results show that (1)S1 and S2 of photosynthesis rate (Pn) and stomatal conductance (Gs) had a increase only slightly at early period, and then resumed its descent which had fluctuation smaller with the course of time, the trend of S3 was declined continuously yet.(2)Intercellular CO2 concentration (Ci) in all treatment groups appears the trend of initially increase sharply and then decrease gradually along with treatment time.(3)Transpiration rate (Tr) in all treatment groups appears the trend of decreased with treatment time progressed and has a slight recover in late treatment. SPAD value and aboveground biomass of alfalfa decreased remarkably with the increasing of salt concentration. Research shows the photosynthesis was restrained and further affected aboveground biomass. At the later stage of low salinity stress, the photosynthesis has recover. The results indicated that alfalfa has a certain salt-tolerant.