Abstract: To investigate whether the mechanical injury on tobacco by topping induces oxidation stress, the contents of superoxide anion(O₂^-·), hydrogen peroxide(H₂O₂)and malondialdehyde(MDA) and the activity of superoxide dismutase(SOD)in leaf tissue were determined and their changes were analyzed and compared with untopped control in a period of time after topping. Bionic signal molecules (BSM) was immediately applied to the plants' top after topping, and relative biochemical indexes were measured. The results showed that topping would stimulate the oxidation stress. Exogenous BSM treatment would inhibit oxidation stress induced by topping, reduce the accumulation of O₂^-·, H₂O₂, MDA in tobacco leaves, and improve the activity of SOD. BSM restricted the synthesis of jasmonic acid by inhibiting oxidation stress, and further reduced the accumulation of alkaloids in topped tobacco plant.