[Objectives]  The objective of this study is to analyze the susceptibility of different larval instars of the beet armyworm Spodoptera exigua (Hübner) to emamectin benzoate and chlorpyrifos and the correlation of susceptibility with enzyme activity. [Methods]  Bioassay was conducted and activity of esterase, mixed function oxidase, glutathione-S- transferase and acetylcholinesterase in different larval instars of the beet armyworm were tested in the laboratory. [Results] Sensitivity to insecticides decreased continuously as larvae developed from 1st to 5th instar, the LC₅₀ for emamectin benzoate of instars 1 to 5 were 0.1010, 0.3561, 0.7568, 1.4325 and 8.4390 mg/L, respectively, while the LC₅₀ for chlorpyrifos were 27.4632, 46.8495, 87.5222, 129.3217 and 1 356.6770 mg/L, respectively. The activities of esterase, mixed function oxidase, glutathione S-transferase and acetylcholinesterase increased with increasing larval instar (the activity of mixed function oxidase of the 5th instar larvae reduced slightly due to the reduced intake of exogenous compounds). There was a negative correlation between enzyme activity and sensitivity to insecticides. [Conclusion]  The sensitivities of different larval instars of beet armyworm to insecticides were found to be significantly different. The best time to control the beet armyworm is at the early instar larval stage, when sensitivity to insecticides is high.