Using ten flue-cured tobacco cultivars, namely NK4, S2007, Yu5, Yu6, 8342, 8302, NC89, Honghuadajinyuan, Y041, Yun 87 and so on, as material, activity changes of nitrate reductase (NR), invertase (Inv), α-amylase(AM) and coordination of chemical constituents in different varieties of flued-cured tobacco when topping and 10, 20, 30, 40 d after topping were studied. The results showed that: 1) Activity of NR and Inv and accumulation of carbohydrates in tobacco plants of Yu5 and NK4 was higher in early maturity stage, followed by continued decreasing. AM activity of Yu6 and NK4 in late maturity stage was higher, making a slow reduction of carbohydrate metabolism intensity,which was conducive to fully mature leaf. 2) Amount of various chemical components of Yu5 and NK4 were in optimal range, with chemical composition in coordination, internal quality prominent. So NK4 and Yu5 planted in Nanyang were more suitable to be cultivated in Nangang tobacco-growing areas.