Alfalfa is an important legume forage with high drought resistance. However, drought stress is still the main adverse factor limiting alfalfa production. A pot-cultivation experiment was performed to investigate the photosynthetic physiology of alfalfa under drought stress. Two Alfalfas varying in their tolerance to drought were used, Longdong with high drought resistance and BL-02-329 with low drought resistance, to systematically study photosynthetic physiology under drought stress. The results showed: (1)Under drought stress, the net photosynthetic rate, transpiration rate, stomatal conductance and chlorophyll content decreased at varing extents, and chloroplast ultrastructure was damaged in the leaves of both Longdong and BL-02-329. Compared to the low drought-resistance BL-02-329, when subjected to drought stress, the net photosynthetic rate in the leaves of high drought-resistance alfalfa, Longdong, decreased more slowly and chloroplast shape and grana structure were less affected.(2)Stomatal restriction was the main factor for the P′n s decrease in the leaves of both alfalfas under light drought stress, while the non-stomatal restriction was the main factor for the P′n s decrease in the leaves of these alfalfas under medium and severe drought stresses.(3) Measurement of fluorescence of PSⅡ Reaction Center indicated Fv/Fm and Fv/Fo decreased in both alfalfas under drought stress. However, when under drought stress, Fv/Fm and Fv/Fo showed a smaller decrease, and the capability to utilize light and the potential to activate PSII remained higher in the high drought-resistance Longdong than in the low drought-resistance BL-02-329.Quenching（qP）of chlorophyll fluorescence decreased and Non-photochemical Quenching（qN）of chlorophyll fluorescence increased for both alfalfas under drought stress. Generally speaking, qP decreased less and qN increased more in the high drought-resistance alfalfa than in the low drought-resistance alfalfa, suggesting that when under drought stress, the electron transport activity and photosynthetic apparatus was damaged less in the alfalfa with high drought-resistance than in the alfalfa with low drought-resistance.