Abstract: 【Objective】To investigate the effects of different concentrations of KNO₃ on the physiological characteristics of T337 seedlings under 100 mmol/L NaCl stress.【Method】Using a potted 2-year-old apple rootstock T337,as the test material,six treatments were established in this experiment,namely CK₁（fresh water irrigation）,salt stress （CK₂）,and salt stress+spraying with four concentrations of KNO₃0.1 （T₁）,0.2 （T₂）,0.3 （T₃）,and 0.4mg/L（T₄）,and the chlorophyll content,photosynthetic fluorescence parameters,antioxidant enzyme activities and osmoregulatory substances of the leaves of each treatment were measured and evaluated using the chlorophyll content,photosynthetic fluorescence parameters,antioxidant enzyme activities,and osmoregulatory substances of each treatment were determined and comprehensively evaluated using principal component analysis.【Result】Photosynthetic pigments （Chl a,Chl b,Chl a/b,Chl a+b） in T337 leaves increased with increasing time under salt stress （CK₂）.Stomatal conductance （G_s）,transpiration rate （T_r）,net photosynthetic rate （P_n）,maximum photochemical efficiency of PSⅡ（F_v/F_m）,and photochemical quenching coefficient （qP） contents all showed a decreasing trend.Nonphotochemical quenching coefficient （qN）,initial fluorescence （F_o）,intercellular carbon dioxide concentration （C_i）,relative conductivity （REC）,malondialdehyde （MDA） content,and proline （Pro） showed an increasing trend,while superoxide dismutase （SOD）,peroxidase （POD）,and catalase （CAT） activities first showed an increasing trend and then a decreasing trend.Compared with CK₁,all indicators of T337 seedlings showed different magnitude of changes after salt stress+exogenous KNO₃ treatment （T₁-T₄）,and there were obvious concentration effects,and the chlorophyll content,G_s,T_r,P_n,F_v/F_m,qP decreased the least in T₃ （0.3 mg/L KNO₃）-treated leaves,C_i,REC,MDA,qN,F_o.The comprehensive evaluation showed that the effects of each treatment on the characteristics of T337 seedlings were in the following order:CK₁＞T₃＞T₁＞T₂＞T₄＞CK₂.The best effect was achieved by 0.3 mg/L KNO₃ treatment to alleviate the damage caused by salt stress to T337.【Conclusion】The application of exogenous KNO₃ at appropriate concentrations can effectively improve the light and ability of T337 seedlings under salt stress,significantly increase the antioxidant enzyme activities and osmoregulatory substances,and reduce cell membrane permeability,thus achieving the effect of alleviating salt stress,and the best effect of 0.3 mg/L KNO₃ treatment to alleviate the damage caused by salt stress on T337.