Abstract: In order to investigate the changes of detoxification metabolic activity in gypsy moth (Lymantria dispar) under the heavy metal pollution stress, we measured the detoxifying metabolism in gypsy moth body feeding on one-year potted seedlings of Populus simonii×P. nigra after treating the soil with three concentrations of copper (Cu, 400, 700, 1 000 mg/kg) and cadmium (Cd, 1.0, 3.0, 6.0 mg/kg). The results showed that Cu and Cd had a significant impact on the detoxification metabolic enzyme in the third and fourth instar larvae. The activities of SOD, ACP and AKP enzymes in vivo in the third instar larvae at medium and high concentrations of Cu and Cd were lower than the control group (P0.05),but at the low concentrations of Cu and Cd, the difference was not obvious (P0.05). The change of activities of POD and GSTs was just the opposite. The activities of POD and GSTs enzymes in the third instar larvae at medium and high concentrations of Cu and Cd increased significantly (P0.05), but not obvious (P0.05) at the low concentrations of Cu and Cd. The activities of SOD, POD, ACP, AKP and GSTs enzymes in the fourth instar larvae at medium and high concentrations of Cu and Cd decreased significantly (P0.05),but not obvious at the low concentrations of Cu and Cd treatment (P0.05). These results showed that the high levels of Cu and Cd stress could significantly interfere with the metabolic detoxification of gypsy moth larvae, and with the third instar larvae developing into the fourth instar, the heavy metal stress finally showed a significant inhibition to the development of larvae, while low levels of Cu and Cd stress had not significant interference with metabolic detoxification of the larvae.