To investigate the photosynthetic efficiency in leaves of Medicago truncatula and its threshold in response to the salt stress in the soil,we used Li-6400 photosynthesis system to measure the photosynthetic parameters of the pot-grown seedlings under different concentrations of salt treatments(0,50,100,150,200,250,300 and 400 mmol·L^-1).The results showed that:(1)The net photosynthetic rate(P_n) and characteristic parameters of photosynthesis in leaves of M.truncatula displayed threshold-value in response to variations in salt concentration.The M.truncatula plants were able to maintain higher photosynthetic productivity and the range of the suitable photosynthetic active radiation(PAR) was 600~1 300 μmol·m^-2·s^-1 at NaCl concentration of 100~200 mmol·L^-1,the maximum of P_n(20.7 μmol·m^-2·s^-1) appeared at NaCl concentration of 150 μmol·mol^-1,and the corresponding PAR was 1 200 μmol·m^-2·s^-1.(2)When the NaCl concentration was lower than 150 mmol·L^-1,the apparent quantum yield(AQY),light compensation point(LCP),respiration rate(R_d) and maximum photosynthesis rate(P_nmax) increased with the NaCl concentration increasing.The maximum AQY(0.030),R_d(0.605 7 μmol·m^-2·s^-1) and P_nmax(19.4 μmol·m^-2·s^-1) as well as the minimum LCP(19.8 μmol·m^-2·s^-1) appeared when treated with 150 mmol·L^-1 NaCl.(3)According to the theory of stomatal limitation,it is the turning point of 150 mmol·L^-1 NaCl,which led to the depression of P_n from stomatal limitation to non-stomatal limitation and the PAR from stomatal limitation to non-stomatal limitation reduced with increasing NaCl concentration.To sum up,M.truncatula had high adaptability to salt stress,and had high photosynthetic productivity in high salt concentration.