Silkworm posterior silkgland (PSG) is a model for studying intracellular trafficking. Here, using this model, we identify several potential cargo proteins of BmKinesin-1 and focus on one candidate, BmCREC. BmCREC (also known as BmSCF) was previously proposed to supercoil DNA in the nucleus. However, we show here that BmCREC is localized in the ER lumen. Its C-terminal tetrapeptide HDEF is recognized by the KDEL receptor, and subsequently it is retrogradely transported by COPI vesicles to the ER. Lacking the HDEF tetrapeptide of BmCREC or knocking down COPI subunits results in decreased ER-retention and simultaneously increased secretion of BmCREC. Furthermore, we find that BmCREC knockdown markedly disrupts the morphology of the ER and Golgi apparatus, and also leads to a defect of PSG tube expansion. Together, our results clarify the ER-retention mechanism of BmCREC, and reveal that BmCREC is indispensable for maintaining ER/Golgi morphology.