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<i>Ultrabithorax</i> (<i>Ubx</i>) is a member of the <i>Hox</i> gene group involved in cell fate decisions, cell proliferation and organ identity. Its function has been extensively researched in <i>Drosophila melanogaster</i> but little is known about it in Lepidoptera. To uncover the function of <i>Ubx</i> in the development of lepidopterans, we constructed the <i>Ubx</i> overexpression (Ubx^OE) strain based on the Nistari strain of <i>Bombyx mori</i>. The Ubx^OE strain showed a small body size, transparent intersegmental membrane and abnormal posterior silk gland (PSG). In the current study, we focused on the effect of <i>Ubx</i> overexpression on the posterior silk gland. As the major protein product of PSG, the mRNA expression of <i>fibroin heavy chain</i> (<i>Fib-H</i>) and <i>fibroin light chain</i> (<i>Fib-L</i>) was upregulated three times in Ubx^OE, but the protein expression of Fib-H and Fib-L was not significantly different. We speculated that the overexpression of <i>Ubx</i> downregulated the expression of <i>Myc</i> and further caused abnormal synthesis of the spliceosome and ribosome. Abnormalities of the spliceosome and ribosome affected the synthesis of protein in the PSG and changed its morphology.