The methodologies of histology and transmission electron microscopy were applied to understand the histological structure and cell type of the accessory nidamental gland (ANG) of adult Sepioteuthis lessoniana, a common species among cephalopods.The results showed that ANG was composed of parietal glandular tissue, glandular tubules and connective tissue.Among them, the parietal glandular tissue was composed of columnar epithelial cells and muscle cells. Glandular tubules were surrounded by monolayer epithelial cells, in which there were abundant organelles including mitochondria, endoplasmic reticulum and golgi complex, etc. Glandular glands were numerous and densely packed.There were a large number of rod-shaped bacteria and secreted substances in the lumen. The space between the gland tubules is filled with connective tissue, in which there were a few blood vessels and muscle fibers. Accordingly, the ANG of S. lessoniana is a secretory gland,the secreted substances include gel-like substances that form egg sheaths and substances that provide nutrients for symbiotic bacteria. The source of symbiotic bacteria belongs to “horizontal transmission”, and its secreted substances have antibacterial activity.