The flue-cured tobacco G80 is not only one of the characteristic varieties in Changsha,but also indispensable raw materials for the series of Baisha cigarette. Defoliation during the late stage of growth seriously reduces the yield and quality of tobacco,and limits the popularization and application of G80. In this paper,in order to investigate the reasons for the defoliation of G80,the transverse section structure,cellulose content and differential proteins of leaf major veins were analyzed. The results revealed that the autolysis and collapse of parenchyma cells is the main cause of defoliation,and fungal infection is a factor inducing the autolysis of parenchyma cells. Application of bud suppression agent is one of the main ways to prevent fungal infection. The low cellulose content,low synthetic ability of xylan and pectin,the low removal ability of abnormal protein and low immune responsiveness are the inner reason for defoliation of flue-cured tobacco G80. The flue-cured tobacco G80 is not only one of the characteristic varieties in Changsha, but also indispensable raw materials for the series of Baisha cigarette. Defoliation during the late stage of growth seriously reduces the yield and quality of tobacco, and limits the popularization and application of G80. In this paper, in order to investigate the reasons for the defoliation of G80, the transverse section structure, cellulose content and differential proteins of leaf major veins were analyzed. The results revealed that the autolysis and collapse of parenchyma cells is the main cause of defoliation, and fungal infection is a factor inducing the autolysis of parenchyma cells. Application of bud suppression agent is one of the main ways to prevent fungal infection. The low cellulose content, low synthetic ability of xylan and pectin, the low removal ability of abnormal protein and low immune responsiveness are the inner reason for defoliation of flue-cured tobacco G80.