Fifty-three cultivable bacterial colonies with significant differences in colony morphologies were isolated from the midguts of field collected cabbage white butterflies (Pieris rapae) in this study. After being cultured in fresh liquid LB medium for 3 days, they were used to smear on artificial diet pellets, which were then fed to 2^nd instar larvae of the fall armyworm (Spodoptera frugiperda). Among them, 9 of the isolated bacterial colonies showed toxicity activity against the larvae. Further bioassays were conducted on the Klebsiella pneumoniae A7 strain, which had the highest insecticidal activity among all the tested bacterial strains. After treatment of 96 hours, the mortality of 2^nd instar S. frugiperda was 26.39% ± 11.98%, 8.33% ± 3.73%, and 56.25% ± 10.26% with the serve of A7 cultured for 1, 2, and 3 days, respectively. All mortality were significantly higher than those of the control group (P < 0.05). The results of whole genome sequencing showed that the total length of the A7 genome was 8.55 Mb, with a GC content of 51.78%, encoding 8148 genes. A7 contains three plasmids with the sizes of 190495 bp, 89806 bp, and 77436 bp, respectively. Further analysis results showed that among the 748 genes encoded by A7 strain, the detection rate of 214 virulence genes was over 70%, that of 306 virulence genes was below 50%, and that of the remaining 228 virulence genes was between 50%~70%. Using VFDB database, 214 virulence genes with > 70% detection rates were annotated into 13 virulence-associated categories. Phylogenetic analysis based on 16S rDNA sequences placed A7 within the Klebsiella sp. clade. This study reports a K. pneumoniae with insecticidal activity against S. frugiperda larvae. Combined with genomic analysis, these findings provide basis for the biological control of S. frugiperda.