Abstract: To determine whether Bemisia tabaci population in transgenic Bt cotton exceeded that in conventional cotton and driven by Bt toxalbumin, and whether this was related to the change in the activity of protective enzymes and detoxification enzymes in B. tabaci, the study used transgenic Bt cotton 'Xinmian33B' and the conventional parental cotton '33' for the case study, and used the ELISA method and enzyme activity determination method to separately analyze Bt toxalbumin content, changes of activities of SOD, CAT and POD protective enzymes and of AChE, CarE and GSTs detoxification enzymes after B. tabaci feeding on transgenic Bt cotton. The results showed that Cry1Ac protein was detectable in B. tabaci after fed for 4 h on transgenic Bt cotton 'Xinmian33B', getting relatively stable after 12 h. The activities of SOD and GSTs were significantly inhibited (P < 0.05) after B. tabaci fed on 'Xinmian33B'. SOD activity steadily declined with the extension of the feeding time, declining 37.8%, 32.1%, 32.0% and 31.9% after feeding for 8 h, 12 h, 24 h and 36 h, respectively. Meanwhile, the activities of CAT, POD and CarE increased significantly (P < 0.05) and the enzyme activity steadily increased with the extension of feeding time. CAT activity was respectively 1.54 times, 1.55 times and 1.42 times; POD activity 1.59 times, 1.39 times and 1.53 times; and CarE activity 1.32 times, 1.34 times and 1.39 times those of the control after feeding for 12 h, 24 h and 36 h. The activity of AChE was not significantly affected after feeding on 'Xinmian33B'. These results suggested that the activities of protective enzymes of B. tabaci increased overall. The activities of detoxification enzymes were significantly affected by transgenic Bt-Cry1Ac cotton. Thus the addition of protective enzyme activity to B. tabaci could increase B. tabaci population. Further study was recommended to determine whether or not this effect was conclusive.