Abstract: The relation between aluminum poisoning and decline of masson pine forest in Nanshan Mountain was studied in both positive and negative aspects by means of the analyses of active aluminum in the forest soil, the soil aluminum releasing test, the aluminum treatment test of the pine （Pinus Massniana Lamb） seedlings and the aluminum soil improvement in the forest land. The results show that there is a great amount of active aluminum in the soil and the active aluminum mainly comes from the activation of mineral aluminum under the acidic condition （pH<4.5）. Aluminum, especially Al³⁺, is poisonous to the growth of the pine. The main reason led to decline of the pine forest was aluminum poisoning. The improvement effects of lime, organic fertilizer and phosphate to the rich-a-luminum soil were obvious.