Abstract: The mode of insecticidal action and the activity of detoxification enzymes (AchE,GSTs,MFO) were studied using three different solvent extracts (acetone,ethanol,chloroform) of Xanthium sibiricum against Lipaphis erysimi and Mythimna separate. The chloroform extracts of X. sibiricum mainly expressed a strong repellency action against L. erysimi: the repellent rate was 94.25% at 4 h with a concentration of 0.05 g/mL,while at 48 h,it was 84.53%; For contact toxicity,the highest mortality rate was 67.24%,and the LC₅₀ was 0.7420 g/mL; Systemic action was extremely weak. For M. separate,the mode of insecticidal action was mainly antifeedant and growth inhibition activity,and the antifeeding corrected mortality rate was as high as 98.27%,and the AFC₅₀ was 0.1985 g/mL; At 72 h,the body weight growth rate was -5.85%,which was significantly different from the CK; Stomach and contact toxicity were weak,and the LC₅₀ were 0.8997 and 1.3070 g/mL respectively. The AchE activity of L. erysimi and M. separata was inhibited by the chloroform extract of X. sibiricum,at 24 h (L.erysimi) and 4 h (M. separate),the inhibition rates of the AchE activity were the highest (37.55% and 25.65%); The activities of GSTs and MFO were not significant.