[Aim]  To clarify the role of Lstoy gene in the development of the compound eyes of Lasioderma serricorne, a global pest of stored foods. [Methods]  The toy gene was obtained by screening L. serricorne transcriptome data, and the open reading frame (ORF) of Lstoy was cloned and analyzed using bioinformatics. The expression levels of Lstoy were detected in different developmental stages and tissues using quantitative real-time PCR (qPCR). RNAi was used to knockdown the expression of Lstoy, after which any subsequent developmental abnormalities, including changes in the expression levels of retinal determination nuclear, and visual, protein genes were observed and recorded. [Results]  The ORF sequence of Lstoy (GenBank accession number: PP796794.1) is 1 380 bp in length, encodes 459 amino acids and has a conserved domain unique to the pax6 gene. Lstoy was continuously expressed in all life stages tested, with highest expression in the adult and pupal stages. Lstoy had the highest expression level in the compound eyes of 5-day-old pupae. RNAi inhibited the expression of Lstoy in prepupae by 54.73%-77.52%. 45.56% of prepupae injected with dsLstoy were unable to complete molt, resulting in malformed compound eye development and death. In 42.22% of prepupae, injection with dsLstoy resulted in molt being delayed by about 2-3 d, slow pigment deposition in the compound eyes, small body size, and ultimately, death. Only 8.89% of prepupae injected with dsLstoy successfully completed molt, but these had abnormal elytra, old epidermis partially adhered to their heads and shrunken compound eyes. All this group also ultimately died. Knock down of Lstoy significantly decreased the expression of five nuclear protein genes (Lseya, Lstsh, Lshth, Lsso, and Lsey) and two visual protein genes (Lslw and Lsuv) by 43.40%-96.94% compared to the control. [Conclusion]  The Lstoy gene plays a crucial role in the molting, metamorphosis, and compound eye development of L. serricorne.