Abstract: Soil acidification has become an ecological problem that is difficult to manage. It is a reasonable means to choose suitable trees for restoration. Picea pungens var. glauca has a certain acid resistance. To explore the physiological mechanism of acid tolerance of young trees of P. pungens var. glauca. The acid stresses of pH 7.0 (CK), 6.0, 5.0 and pH 4.0 were treated for 4 weeks in P. pungens var. glauca. Then the photosynthesis characteristics, leaf organic osmotic adjustment substances and antioxidant enzyme activities of the young saplings were measured. The results showed that when pH≥6.0, the photosynthetic parameters (net photosynthetic rate, transpiration rate, intercellular CO₂ concentration and stomatal conductance) of plants showed an upward trend; when pH<6.0, the photosynthesis parameters showed a downward trend; when pH≥6.0, the concentrations of free proline and soluble protein in leaves increased; the soluble sugar content decreased with decreasing pH throughout the experimental pH range; when pH≥6.0, the activities of peroxidase (POD) and superoxide dismutase (SOD) showed an upward trend; when pH<6.0, both activities showed a downward trend; the change in malondialdehyde (MDA) content is contrary to the other two. Based on the above analysis, it suggested that the physiological regulation threshold of acid tolerance of saplings may be about pH 6.0, which can grow better under mild acidic conditions. Strong acid should be avoided to cause harmful effects in P. pungens var. glauca planting.