Benign tumors of the breast are most commonly seen in young women aged 20-25 years. In contrast, mammary malformation tumor is a rare benign tumor of the breast. Since it was first reported abroad in the 1970s, the incidence rate in China is 0.12%-0.16% of solid breast tumors, mostly occurring in late lactation and early menopause, and the etiology is still unclear. Recently, we successfully removed a giant mammary malformation tumor from the left breast of a 38-year-old patient. The patient, XX, was found to have bilateral breast asymmetry without distension and pain after childbirth, and had a history of more than 10 years, but because the left breast was combined with nipple invagination, it was not corrected during breastfeeding, and it was always thought that the bilateral breast asymmetry was caused by improper breastfeeding and was not treated. Recently, she felt that her left breast was enlarged again, and the outer part of it was obvious, and the texture was hardening. After completing the preoperative examination, we proposed to perform excision of the mass and intraoperative frozen biopsy. In order to maintain a good appearance of the breast, an arcuate incision was chosen in the areola, and a large mass with medium texture and intact envelope was seen in the lower to outer quadrant of the left breast. Breast malformation tumor”. After treatment of the inframammary ducts and correction of nipple invagination, the affected breast was reshaped and sutured. Postoperatively, the size and shape of both breasts were basically symmetrical. Due to the low clinical awareness of mammary malformation tumor and the diversity of its tissue components, the diagnosis is usually not clear based on breast ultrasonography alone, but mostly on pathology after surgery. Although breast malformation tumor is a rare benign breast tumor, it can be completely excised by surgical treatment without recurrence after surgery. The incidence of breast tumors is increasing among women today, so early detection is especially important. As a specialist, we remind women to learn breast self-examination and seek early medical consultation when suspicious conditions are detected.