Haedoxan A (HA) and Phrymarolin I (PI) are dioxabicyclo lignans from medical plant Phryma leptostachya. Previous studies showed that HA has remarkably insecticidal activity against some insect pests, and Phrymarolins could enhance the effectiveness of some synthetic insecticides. However, whether PI produces a synergistic effect on HA remains unknown and its synergism mechanism is still unclear, as well as the information regarding the metabolism of HA. In this study, we used triphenyl phosphate (TPP), diethyl maleate (DEM), and piperonyl butoxide (PBO) to detect the metabolic activity of HA and the synergistic effect of PI on HA in Mythimna separata. After treated with PBO, TPP, and DEM at 70 μg/mL by the method of topical application, the stomach toxicity LC50 value of HA on Mythimna separata larvae was declined from 27.06 μg/mL to 6.7, 9.32, and 7.54 μg/mL, respectively. In leaf-disc bioassay, the stomach toxicity LC50 value declined to 9.89, 14.16, 15.86, and 11.89 μg/mL, respectively, with the addition of PBO, TPP, DEM, and PI at 70 μg/mL. Another necessary consequence was that the synergism ratios of PI to HA, permethrin, indoxacarb, and avermectin were 2.28-fold, 6.08-fold, 4.23-fold, and 3.38-fold, respectively, which were approximate to that of PBO. Moreover, further enzymatic activities test showed that PI dramatically inhibited the cytochrome P450 activity in comparison with that of control. Taken together, these results indicated that three metabolic enzyme families, especially the monooxygenases, play an important role in detoxifying HA. Additionally, PI used as a potential source of botanical synergist is promising.