Abstract: Based on the indoor virulence and field control test results showed that the mixture of Bacillus thuringiensis（Bt） and chlorbenzuron had strong synergism to the second and third generation of the larvae of Hyphantria cunea. This paper analyzed the influence on the activity of midgut chitinase and cuticle AChE on the 4th instar larvae of Hyphantria cunea the result as following: As the concentration increased and treatment time prolonged 5 concentrations of the mixture of Bt and chlorbenzuron treated, the midgut chitinase activity of the 4th instar larvae of Hyphantria cunea showed inhibitory effect more obvious and got significant differences （P<001）. It indicated that Bt+ chlorbenzuron mixture could improved the effect to the 4th instar larvae of Hyphantria cunea. As the treatment time increased 5 concentrations of the mixture of Bt and chlorbenzuron treated, the cuticle AChE activity of the 4th instar larvae of Hyphantria cunea increased first and then decreased. From 36-96 h the mixture induced the trend of the cuticle AChE activity gradually increased and treatment to 120h the cuticle AChE activity was significantly diminished. The inhibitory rate of the cuticle AChE activity was 20.13%-9000%. After the cuticle AChE activity decreased the 4th instar larvae of Hyphantria cunea died quickly （P<001）. It was another reason of the synergistic effect of the mixture.