Abstract: The toxicity of spinosad to beet armyworm at different developmental stages was determined. Spinosad could significantly reduce the adult fecundity, adult lifetime and survival rate of neonate larvae. The survival rate of the neonate larvae of eggs treated with 6.25 μg/mL spinosad was only 7.48%, relative to the control of 76.72%. The fecundity and lifetime of adults fed with 12.5 μg/L spinosad were decreased to 1/6 and 1/2.5 of the control fed with honey solution. With the larval development, the sensitivity to spinosad decreased significantly, the LC50 of the third and fourth instar larvae to spinosad increased more than 9 and 16 times than that of second instar larvae. PBO and TPP have no synergism for spinosad and this insecticide also did not affect the hatchability of treated eggs, pupal weight and eclosion rate of treated larvae. There was no significant difference in the activities of superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD) in vivo in the third instar larvae between the control and the treatment with sublethal dose of spinosad within 0~24 hours, it seems there was no obvious relationship between the sensitivity of beet armyworm to spinosad and the activities of endogenous enzymes of protective system.