Abstract: In this paper, structural modification of benzoylurea compound NK-17, which has been reported to have excellent insecticidal activity, resulted in the compound (E)-N-((4-((tert-butoxyimino)methyl)phenyl)(methyl)carbamoyl)-2,6-difluoro benzamide (hereinafter referred to as HN-21 )to improve its solubility in organic solvents. The leaf film method was used to determine the bioactivity of lufenuron and HN-21 against the second instar larvae of Spodoptera frugiperda, and their effects on the chitin synthesis process were also analyzed. The results showed that the median lethal concentration (LC₅₀) of HN-21 for 72 h was 0.681 mg/L, significantly lower than that of the positive control lufenuron (10.052 mg/L), and all the abnormal larvae died during molting. Compared to the control group, the expression of chitin synthesis pathway genes (sfTRE1, sfTRE2, sfGFAT, sfUAP, sfCHS1) decreased significantly after 24 h of lufenuron treatment at the concentration of 10.052 mg/L, earlier than HN-21 treatment at the concentration of 0.681 mg/L, but there was no significant difference between the two groups after 48 h treatment. These results can provide a theoretical basis for the development and utilization of benzoylurea compounds.