[Objectives]  To investigate the synergistic mechanism responsible for the increased insecticidal activity of Cry1Ac when combined with the PxCAD1 peptide. [Methods]  The leaf dip bioaasay method was used to treat leaves fed to a randomly selected group of 3rd instar Plutella xylostella larvae with a mixture of the PxCAD1 peptide and Cry1Ac toxin, after which their larval mortality, pupation rate and emergence rate were recorded and compared to those of a control group. In addition, trends in Carboxylesterase, Glutathione-S-transferase, total protease and trypsin-like activity, were measured and compared between groups. [Results]  The mortality rate and emergence rate of larvae fed leaves treated with the PxCAD1 peptide and Cry1Ac toxin mixture were significantly higher than that of the control group, and their pupation rate was also lower. The carboxylesterase activity of treatment group larvae was significantly higher than that of the control group after 24 h and 48 h, then significantly decreased after 72 h. Glutathione-S-transferase activity was significantly higher after 24 h, then decreased significantly after 48 h with no further change after 72 h. Total protease activity was significantly lower than that of the control group only at 72 h, and there was no significant change in tryptase activity. [Conclusion]  PxCAD1 not only has a significant synergistic effect on Cry1Ac, but also has an adverse effect on the growth and development of P. xylostella larvae and the activity of Carboxylesterase, Glutathione-S-transferase and total protease in vivo. These results facilitate further study of the synergistic effects of midgut receptor proteins on Cry1Ac.