Abstract: To clarify the relationship between flue-tobacco toughness and internal quality,and to optimize the evaluation index system of flue-cured tobacco appearance quality,we collected upper and middle parts flue-cured tobacco leaf samples from six ecological secondary regions in China.Correlation and Random forest regression analysis were used to study the relationship between tobacco leaf toughness and main chemical components and sensory quality indexes.The main results were as follow:Obvious regional differences in leaf flexibility were observed.The flue-tobacco toughness score of southeastern Chongqing was good,followed by western Henan and southern Hunan,while eastern Yunnan and central Henan was generally average;Toughness was significantly or extremely significantly positively correlated with reducing sugar (RS) and total sugar content (TS),the ratio of reducing sugar and total sugar (RS/TS) and the ratio of reducing sugar and nicotine (RS/Nic);The toughness score of tobacco leaves was significantly positively correlated with aroma quality,aroma quantity,odor,irritation,dryness and return sweetness and other sensory indexes;Random forest regression analysis showed that the contribution rate of toughness to the variation of RS,aroma quality and total sensory score was higher than that of other appearance indexes.Therefore,toughness can better represent the intrinsic quality of flue-cured tobacco leaves and play an important role in simplifying the assessment indicators of flue-cured tobacco leaf quality.