[Objectives]  To investigate the effects of infection with the nucleopolyhedrovirus (NPV) and diet on the midgut of beet armyworm (Spodoptera exigua) larvae. [Methods]  Peritrophic matrices (PMs) were dissected from larvae at different times post-infection so that change in this tissue could be observed over time. Pathological changes in midgut tissues of NPV-infected and non-infected larvae feeding on artificial diets and different plants were revealed by making histological sections. [Results]  Infection with NPV caused the PM to change from a transparent, flexible, cylindrical structure to an opalescent, sheet-like structure with reduced elasticity, and ultimately, a group of inelastic fragments with no structural integrity. Diet had no significant effect on the midgut tissues of uninfected larvae. However, diet had obvious effects on the midguts of infected larvae. In some diet treatment groups intestinal epithelial cells became elongate and then irregular, the intestinal wall thickened, intercellular spaces became larger, the cellular arrangement became disordered with cells detaching, ultimately causing the structure of the PM was to be seriously damaged or even disappear. There was a significant interaction between NPV infection and diet with NPV having the greatest effect on the midgut tissue of larvae that were fed artificial diets and soybeans (Glycine max), followed by those fed collards (Brassica oleracea), and the least effect on those fed water spinach (Ipomoea aquatica). [Conclusion]  NPV had significant adverse effects the midgut tissue of beet armyworm larvae but the severity of these effects varied with diet.