To elucidate the function of the insect intestinal mucin gene HaIIM5AC in Helicoverpa armigera (the cotton bollworm) during infection by the Helicoverpa armigera nucleopolyhedrovirus (HearNPV). [Methods]  Significantly up-regulated HaIIM genes post-HearNPV infection were identified based on previously generated transcriptome data from healthy and HearNPV-infected H. armigera larvae. The expression level of HaIIM5AC in the midgut of healthy and infected larvae (24, 48, 72, 96 h post-infection [hpi]) was analyzed using quantitative real-time PCR (qPCR). RNA interference (RNAi) via dsRNA injection was employed to knock down HaIIM5AC expression in infected larvae. Mortality rates of infected larvae were recorded, and viral load was quantified using qPCR to investigate the role of HaIIM5AC in defense against HearNPV infection. In addition, paraffin sectioning and hematoxylin-eosin (HE) staining were used to observe peritrophic membrane (PM) morphology. [Results]  Comparative transcriptome analysis of healthy and infected larval midguts found that the HaIIM5AC gene had been significantly up-regulated 24 and 48 hpi (P < 0.001). Compared to healthy larvae, HaIIM5AC expression was significantly up-regulated in the midgut of infected larvae at all time points (24 hpi: P < 0.001; 48 hpi: P < 0.001; 72 hpi: P < 0.01; 96 hpi: P < 0.001). RNAi results showed that HaIIM5AC expression was reduced by 54.72% and 43.48% 24 and 48 hours after the injection of dsHaIIM5AC, respectively. Silencing HaIIM5AC resulted in significantly accelerated mortality rates in infected larvae (P < 0.05), significantly increased viral replication levels (P < 0.001), and obvious structural damage to the PM. [Conclusion]  The HaIIM5AC gene defends against HearNPV infection by maintaining the integrity of the PM, playing a crucial role in the immune response of H. armigera to HearNPV infection.