Abstract: In order to determine the influence of lower leaves harvest frequency on the carbon nitrogen metabolism, senescence, yield and quality in flue-cured tobacco upper leaves. With Yunyan 87 as an experimental material, four kinds of lower leaves harvesting frequency were designed. The carbon-nitrogen metabolism related enzymes and products, peroxide index and related enzymes, nicotine content of root wound fluid, economic character, conventional chemical composition and sensory quality were analysized. The results show that with the lowest leaf harvest times increase, the activity of nitrate reductase (NR) which in the upper leaves was decreased, and the activity of antioxidant enzymes such as superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) were decreased, the content of malondialdehyde (MDA) was increased. The oxidative aging of the leaves was increased, nitrogen metabolism was decreased, and the content of nicotine was increased. The advisable harvesting frequency was 4, the upper leaves' nitrogen assimilation metabolism was weakened, carbon assimilation metabolism was vigorous, and cured leaves' sugar content were higher, reducing sugar/nicotine were more coordination, sensory quality and economic traits were better than others.