With 2year old apple dwarf rootstock M9T337 as test material, using pot experiment method, we set watering water (CK) and saltalkali stress (0.1mol/L NaCl+NaHCO₃ solution), followed by spraying five concentrations of H₂O₂ treatment ［0 (T₁), 0.2 mmol/L (T₂), 0.4 mmol/L (T₃), 0.6 mmol/L (T₄), 0.8 mmol /L (T₅)］. The chlorophyll content, photosynthetic gas exchange parameters, osmotic adjustment substance contents, antioxidant enzyme activities and cell membrane permeability of each treatment were measured, and comprehensive evaluation was conducted by correlation and principal component analysis, so as to explore the physiological mechanism of exogenous hydrogen peroxide (H2O₂) enhancing salinealkali tolerance. The results show that: (1) the contents of chlorophyll a (Chl a), chlorophyll b (Chl b), total chlorophyll (Chl t), net photosynthetic rate (P_n), stomatal conductance (G_s), transpiration rate (T_r) and soluble protein (SP) content of M9T337 seedlings decreased gradually with the extension of salinealkali stress. Intercellular CO₂ concentration (C_i), soluble total sugar (TSS) content, proline (Pro) content, catalase (CAT) activity, ascorbate peroxidase (APX) activity, relative electrical conductivity (REC) and malondialdehyde (MDA) content all showed an increasing trend. The activities of superoxide dismutase (SOD) and peroxidase (POD) increased first and then decreased.(2) Compared with CK, each index of M9T337 seedling leaves changed in different amplitude after treatment with exogenous H2O₂, and there was an obvious concentration effect. The decreases of Chl a, Chl b, Chl t, SP and G_s in leaves treated with T₃ (0.4 mmol/L H₂O₂) were the smallest, and the increases of C_i, REC and MDA were the smallest. TSS, Pro and APX increased the most. (3) Leaf P_n of M9T337 seedlings was positively correlated with T_r, G_s, Chl a, Chl b, Chl t, SP, SOD and POD, and negatively correlated with C_i, MDA, CAT, APX and REC. (4) Comprehensive evaluation showed that the effect of each treatment on the physiological characteristics of M9T337 seedling leaves was in the order of CK>T₃>T₄>T₂>T₅>T₁. The results showed that the suitable concentration of H₂O₂ could effectively improve the photosynthetic capacity of M9T337 seedlings under saltalkali stress, significantly increase the activities of antioxidant enzymes and the contents of osmotic regulatory substances, and reduce the membrane permeability, so as to alleviate the saltalkali stress, and 0.4 mmol/L H₂O₂ treatment had the best effect.