Trees can effectively absorb and block poisonous gases, such as SO₂ and NO_x, when they are growing. But they can also in-crease the content of poisonous gases in forest and its near area owing to interception. The contents of SO₂ and NO_x in and near the forest were the highest during the growth period of leaf and branch, and were 5.2 and 6.7 times respectively the content during the growth of height and diameter. In the growth period of leaf and branch, the content of SO₂ was higher in the morning and lower in the afternoon, and the content in the morning was 4 times higher as much as in the afternoon. It also showed charicteristic of being highest in the forest belt and lowest outside the forest belt, but the content of NO_x was lowerest at noon. In the growth period of height and diameter, contents of SO₂ and NO, in the atmosphere were high in the morning and were 3-5 times higher than that in the afternoon. In slowly growing period, the contents of SO₂ and NO_x changed little. The content of SO₂ increased continuously from the morning both in and outside the forest belt, the content of NO_x increased more rapidly outside the forest belt, and varied litde in forest belt and inside of forest belt. The peak period of growth of height and diameter was preferred for rest, tour and bodybuilding.