Fallarmyworm, Spodoptera frugiperda, is one of the most devastating invasive pests in China. Chlorantraniliprole (CH) is currently the main agent for controlling S. frugiperda. Carbaryl (CA) has been widely used as a foliar treatment to control S. frugiperda, although the pest has become highly resistant to it. This study investigates the comparative toxicity and joint effects of CH and CA on S. frugiperda. Time-toxicity results showed that CH had high toxicity to 1st and 3rd instar larvae, whereas CA had very low toxicity to 1st and 3rd instar larvae. The mixtures of CH and CA at different mass ratios showed strong synergistic effects on toxicity, and the mass ratio of 2:1 exhibited the highest toxicity to S. frugiperda. Furthermore, the synergistic toxicity of CH and CA at the 2:1 mass ratio (CH+CA) was also verified in field populations of S. frugiperda. The life-history parameters showed that CH+CA dramatically decreased the survival rate and fecundity of the parent population (F0) compared with CH treatment at the same concentration. Besides, CH and CH+CA mixture showed induction effect on cytochrome P450s and glutathione-S-transferases (GSTs) activities in S. frugiperda, with cytochrome P450s enzyme responding the fastest. In conclusion, this research found CH+CA provided synergistic effects on the toxicity and the sublethal effect on larvae. The joint effects on the life-history parameters and the detoxifying enzymes in S. frugiperda, may be useful for implementing IPM programs against this Lepidoptera pest.