Abstract: The effect of topping time and external IAA on tobacco metabolism, yield, and quality were studied. The results showed that changes was insignificant for total carbon，total nitrogen, and main compound of nitrogen after different treatment. Contents of IAA, GA and dissolubility of sugar increased after early topping, on the other hand, content of ABA decreased, the duration of starch accumulate were extended. Tobacco maturity senescence and quality building were influenced by different measures, because the balance of carbohydrate metabolism was regulated. After T1A treatment, average and finest grade proportion increased, yield and quality of tobacco leaves were improved.