Abstract: In order to screen for safer insecticides to Aphidius gifuensis Ashmead and know about the effects of conventional insecticides on protective enzymes of Aphidius gifuensis Ashmead in tobacco fields, toxicity of three conventional insecticides to Aphidius gifuensis Ashmead was determined via the residual film method. Effects of different concentrations of imidacloprid on SOD, POD and CAT activities of Aphidius gifuensis Ashmead was also determined. The results showed that the order from high toxicity to low toxicity was: acetamiprid >lambda-cyhalothrin >imidacloprid. In terms of the effects of different concentrations of imidacloprid on the life activities of Aphidius gifuensis Ashmead and their defense mechanisms, all the treatments showed inhibition on enzyme activities except that the LC₁₀ concentration of imidacloprid improved POD activities in adults and CAT activities increased significantly in adults after 12h treatment of three concentrations. This result provides theory basis for understanding the defense mechanisms of Aphidius gifuensis Ashmead against insecticides.