The cotton bollworm Helicoverpa armigera (Hübner, 1808) (Lepidoptera: Noctuidae) is a globally distributed polyphagous pest that damages various crops. Pesticides remain a primary and effective strategy for controlling cotton bollworm populations. This study evaluated the effects of broflanilide on H. armigera by integrating life-table analysis, detoxification enzyme activity assays, and gut microbiome characterization. Broflanilide showed high toxicity against third-instar larvae. Parental LC₅₀ exposure prolonged larval development and reduced larval survival in the F₁ generation. In addition, parental exposure to both LC₃₀ and LC₅₀ reduced adult emergence, fecundity and major population growth parameters, especially the intrinsic rate of increase (r) and net reproductive rate (R₀), demonstrating a significant transgenerational inhibitory effect. Enzyme assays showed that carboxylesterase (CarE) activity was induced after broflanilide exposure, glutathione S-transferase (GST) activity showed a time-dependent response with the strongest induction generally observed under LC₁₀ treatment, whereas cytochrome P450 monooxygenase (P450) activity was generally inhibited. Gut microbiota analysis showed that LC₅₀ exposure significantly reduced the relative abundance of Enterococcus. Functional prediction further indicated enrichment trends in pathways related to xenobiotic degradation and metabolism. Overall, broflanilide not only exhibited strong lethal activity against H. armigera, but also produced sustained effects on population fitness, detoxification metabolism and gut microbial ecology. These findings provide a theoretical basis for the rational use of broflanilide and improve our understanding of its sublethal physiological and microbial effects on H. armigera.