The oriental armyworm Mythimna separata Walker is a serious threat to the production of cereals. Its control has largely relied on synthetic insecticides, which led to the decrease in their effectiveness. In China, cantharidin, a natural compound of insect origin with a mode of action different from a conventional insecticide, is being developed as a bio-insecticide for the control of lepidopteran pests. Its toxicological effects have already been studied in M. separata. However, its sublethal effects on physiological and population parameters have not yet been studied. The leaf dip bioassay results showed that cantharidin-AC (cantharidin acetone solution) had a high level of toxicity against M. separata and the 96 h LC₅₀ value was 223 µg/ml. The sublethal effects of cantharidin exposure for 72 h at LC₁₀ (77 −g/ml) on physiological and population parameters of M. sepatata were also investigated, and data were subjected to an age-stage two-sex life-table. The sublethal effects of cantharidin indicated reduction in survival rates of larval, pupal, and adult stages. In addition, both male and female moths were observed with crippled wings in the cantharidin-treated cohort. The mean values of the finite rate of increase (λ), the intrinsic rate of increase (r), and the net reproductive rate (R0) were significantly lower in the treatment than in the control. The fecundity was also strongly affected by a sublethal cantharidin concentration. A sublethal concentration of cantharidin may reduce the population growth of M. separata by decreasing its survival and reproduction and by increasing its generation time.