Abstract: To study the effects of bionics signal molecule (BSM) on oxidative stress of different flue-cured tobacco varieties, Yunyan87, Cuibi-1 (CB-1) and K326 were used. Plants were treated with not topping (CK), topping (T1) and topping-BSM (T2). The contents of superoxide anion (O₂^-.), hydrogen peroxide (H₂O₂), malondialdehyde (MDA) and the activity of superoxide dismutase(SOD) in flue-cured tobacco were measured. The results showed that, compared with CK, the contents of O₂^-.and H₂O₂ increased after 6 h of topping in Yunyan87, Cuibi-1(CB-1) and K326. The SOD activity and the MDA concentration increased after 96 h, which entailed that mechanical damage, e.g. topping, could result in plant oxidative stress. In the treatment of BSM after topping, the contents of O₂^-., H₂O₂ and MDA in Yunyan87 increased, but the activity of SOD decreased. However, the contents of O₂^-., H₂O₂ and MDA in CB-1 and K326 decreased, but the activity of SOD increased. Therefore, BSM could effectively alleviate the oxidative stress caused by topping in CB-1 and K326, but not in Yunyan87.