Abstract: This paper deals with relation between tree stomatal infiltration and SO₂ injury and the protection effect of ABA. Experiments of SO₂ smoking to the selected trees were carried out and the data were collected in the field. The results show that stomatal infiltration was a comparably constant index for certain tree species. The infiltration of trees is negatively correlated with the combine extent of chlorophy, but not obviously. The index is also positively correlated with K⁺ efflux in leaf(r=0.92, α<0.01). In the experiment of SO₂ smoking, the effect on infiltration of same species under different SO₂ concentration was little, less than one grade, while K⁺ efflux increased with the increment of SO₂ amount absorbed by the leaves. When the leaves were sprayed with ABA solution, the higher the ABA solution concentration was, the lower the K⁺ efflux was. When leaves sprayed with ABA solution were smoked with 2.5 mol·L^-1 for four hours, the infiltration of leaves with 30 mol·L^-1 ABA solution dropped 1.5～3 and K⁺ concentration decreased 36.5%～54.8% on leaves. It indicates that the ABA solution on leaves has remarkable effect of protection of SO₂ injury.