In recent years, the interactions between baculoviruses and their insect hosts have become a research focus because baculoviruses can suppress the development and manipulate the behavior of insects. Many studies reported that nucleopolyhedrovirus (NPV) infection might disrupt the hormone balance in insects, but the effect of NPV infection on the development and expression of hormone-related genes in larvae of Helicoverpa armigera Boddie (Lepidoptera: Noctuidae) remains unclear. In this study, the mortality, development time, and pupal weight of H. armigera were recorded after 4th and 5th instars had been treated per os with different concentrations of Helicoverpa armigera single NPV (HaSNPV). Results showed that mortality increased and development time was prolonged to different degrees along with increasing concentrations of HaSNPV. The pupal weight did not differ between the HaSNPV-infected and control insects when 4th instars were infected but was significantly reduced when 5th instars were infected with HaSNPV at concentrations of 10⁹ and 10¹⁰ polyhedral inclusion bodies (PIB) per milliliter. Compared with the healthy control group, larval body weight was significantly reduced from the 3rd day after infection when 4th instars had been treated with HaSNPV at concentrations of 10⁸ and 10⁹ PIB/mL. Results from quantitative reverse-transcriptase polymerase chain reaction assays revealed that 20-hydroxyecdysone—related genes (ECR, USP, E75, and NTF2) were down-regulated and juvenile hormone—related genes (MET, JHi, and HSP90) were up-regulated after HaSNPV infection. This study improves our understanding of the interactions between baculoviruses and host insects.