Abstract: 【Objective】The effects of nitrogen on the midrib characteristics and curing characteristics of tobacco leaves and the relationship between the midrib characteristics and curing characteristics of tobacco leaves were studied to provide a theoretical basis for optimizing the curing process of tobacco leaves within a reasonable range of nitrogen application.【Method】The upper leaves of flue-cured tobacco variety Yunyan 87 were used as the test material, three nitrogen application rates were set as 75 kg/ha(low nitrogen), 105 kg/ha(normal) and 135 kg/ha(high nitrogen).The water migration rate of midrib of fresh tobacco leaves, the hardness, lignin content and moisture content of midrib, leaf moisture, chlorophyll content and polyphenol oxidase(PPO) activity during curing were studied, and the economic characters of cured tobacco leaves were counted.【Result】Water transfer rate of midrib of fresh tobacco leaves:low nitrogen< normal< high nitrogen.In the curing process, the midrib of low-nitrogen tobacco leaves lost less water, and the rate of water loss in the leaf-drying stage(48-96 h) was 0.07%/h.The ratio of bound water to free water was large, the hardness of midrib was small and changed slowly, and the lignin content was low.The water loss rate of the midrib with normal nitrogen fertilizer was 0.22%/h in the leaf-drying stage.The bound water to free water ratio, hardness and lignin content were all in the middle.The midrib of high-nitrogen tobacco leaves lost more water, and the rate of water loss was 0.35%/h in the leaf-drying stage.The ratio of bound water to free water was smaller, the hardness of midrib was larger and the change rate was faster, and the lignin content was higher.In the curing process, the water loss rate of low-nitrogen tobacco leaves was 0.20%/h in the yellowing stage(0-48 h).The chlorophyll degradation amount was 89.47%, and the degradation rate was 1.24%/h, indicating that the easy curing potential was good, high PPO activity, indicating that the endurable curing potential was poor.The proportion of variegated tobacco leaves after curing was up to 12.47%.The water loss of normal nitrogen tobacco leaves was in the middle, and the water loss rate in the yellowing stage was 0.13%/h.The chlorophyll degradation amount was 87.35%, and the degradation rate was 1.21%/h, indicating that the easy curing potential was medium, the activity of PPO was in the middle, indicating that the endurable curing potential was medium.The rate of superiors tobacco leaves after curing was up to 31.27%.The water loss rate of high-nitrogen tobacco leaves was small, and was 0.08%/h in the yellowing stage.The chlorophyll degradation amount was 85.64%, and the degradation rate was 1.19%/h, indicating easy curing potential was poor, low PPO activity, indicating endurable curing potential was good.The proportion of green tobacco leaves after curing was up to 14.50%.The correlation analysis showed that there was extremely significant correlation between the midrib indexes and the curing characteristic indexes of tobacco leaves(P< 0.01), except that the high nitrogen tobacco leaves, which has significant correlation between the midrib hardness and the PPO activity of leaves(P< 0.05).【Conclusion】Nitrogen has significant effect on the texture and physiology of tobacco leaves.Under the nitrogen rates of 105 kg/ha, the development of midrib and their water conservation are better.During the curing process, the regulating effect of midrib on leaf water loss is better, thus yellowing and water loss of tobacco leaves are more harmonious and endurable curing potential is good, it can provide reference for the optimization of tobacco curing process with different nitrogen application rates in the appropriate range.