In order to provide a theoretical basis for introduction and cultivation of Koelreuteria paniculata in saline alkali areas, this paper studied effects of mixed salt stress on the photosynthesis and chlorophyll fluorescence characteristics of K. paniculata seedlings and its adaptation mechanism. The paper performed pot experiment at the six gradients of 0 (CK), 100 (S₁₀₀), 200 (S₂₀₀), 300 (S₃₀₀), 400 (S₄₀₀) and 500 (S₅₀₀) mmol·L^-1 , which were prepared by mixing NaCl and NaHCO₃ at molar mass ratio of 1∶1, and 2yearold K. paniculata cuttings were used in the pot experiment. The results showed that: (1) with the increase of stress degree, K. paniculata seedlings showed some apparent symptoms, such as yellowing, scorching, curling, root reduction, plant death and so on. (2) With the increase of stress degree, leaf length increment and leaf width incremen of K. paniculata seedling increased. The leaf length increment and leaf width increment of K. paniculata seedling were no difference with the control group when mixed salt concentration was 100 and 200 mmol·L^-1 , but the leaf length increment began to decrease significantly when mixed salt concentration reached 300 mmol·L^-1 ，and the leaf width increment began to decrease significantly compared with control group when mixed salt concentration reached 400 mmol·L^-1 . (3) The chlorophyll, PSⅡ actual quantum (Yield), photochemical quenching coefficient (qP) and maximum quantum yield of PSⅡ(F_v/F_m) of K. paniculata seedling leaves were no difference with the control group when mixed salt concentration was 100 mmol·L^-1 , but the indexes except maximum quantum yield of PSⅡ(F_v/F_m) began to decrease significantly when mixed salt concentration reached 200 mmol·L^-1 , and the indexes except maximum quantum yield of PSⅡ (F_v/F_m) began to decrease significantly compared with control group when mixed salt concentration reached 300 mmol·L^-1 . (4) With the increase of mixed salt stress, net photosynthetic rate (P_n), stomatal conductance (G_s) and transpiration rate (T_r) of K. paniculata seedlings decreased as a whole; When mixed salt stress was 0-30 days, and mixed salt concentration was 100-300 mmol·L^-1 , changes of net photosynthetic rate (P_n), intercellular CO₂ concentration (C_i) and stomatal conductance (G_s) of K. paniculata seedling leaves were same. With the increase of stress degree, net photosynthetic rate (P_n) and stomatal conductance (G_s) decreased, and intercellular CO₂ concentration (C_i) increased. These results showed that: when the mixed salt concentration was 100 mmol·L^-1 , the growth indexes and physiological indexes of K. paniculata did not change significantly, showing a certain ability to tolerate mixed salt stress. At this time, stomatal restriction was the main factor. The growth indexes, chlorophyll content and photosynthetic physiological indexes of K. paniculata seedling leaves decreased significantly as a whole when mixed salt concentration reached 200 mmol·L^-1 , resulting in the inhibition of plant growth. At this time, nonstomatal restriction and photochemical activity inactivation were main factors affecting photosynthesis.