Abstract: Through measuring photosynthetic characteristics and chlorophyll fluorescence parameters of Betula platyphylla under salt stress to study its salt tolerance,we would provide the basis for salt tolerance birch screened.Six halfsib families of Betula platyphylla as test material,with 04 percentage NaCl irrigation methods and applying stress once every 3d,changes of photosynthetic characteristics and chlorophyll fluorescence parameters under salt stress were researched.The results showed that under salt stress, the relative content of chlorophyll increased first,and then decreased,yet net photosynthetic rate declined, which was resulted from stoma limitation and nonstoma limitation.Chlorophyll fluorescence parameters Fv/Fm increased at first and then reduced,φ PS Ⅱ and qP dropped, yet qN remained constant or fluctuated obviously.In conclusion,the relative content of chlorophyll of MP1 and MP2 family were high and had small change ranges, moreover,net photosynthetic rate,Fv/Fm,φPSⅡ and qP declined slightly,while qN remained stable,so MP1 and MP2 family showed more tolerant to salt than others.