Abstract: In order to reduce nicotine conversion rate and improve tobacco quality, burley tobacco B37 plants of different nicotine conversion rates were treated by two ways, topping only and topping followed by bionic signal molecule (BSM) application. The activity of SOD and the contents of MDA and alkaloid of all samples were detected respectively, and the changes in alkaloid proportion and nicotine conversion rates were analyzed. The results showed that:topping increased the SOD activity in tobaccos leaf, particularly in the leaves of high-converters. BSM treatment also increased the activity of SOD, however it increased in high-converters at a lower rate. Topping increased the content of MDA in tobacco leaves, but its increase rate in high-converters was lower. BSM treatment decreased the content of MDA in leaf, particularly in the leaves of high-converters. BSM decreased the contents of total alkaloid and the four major alkaloids, particularly the content of nornicotine.BSM affected the nicotine proportion in upper and middle leaves of non-converters slightly; but affected heavily high-converter B37-38, in the upper and middle leaves of B37-38, the nicotine proportion raised by 15.52 and 10.40 percentage points, while nornicotine proportion reduced by 15.16 and 10.03 percentage points, respectively. From the non-converter to high-converter, the effect of BSM on the decrease of nornicotine proportion increased. The effects of BSM application were more significant on high-converters.