Plasmacytoid mastitis is caused by obstruction and dilatation of the breast ducts from various causes, with overflow of lipid secretions from the ducts into the periductal tissue and localized microscopic changes with plasma cell infiltration as the main change. Plasmacytoid mastitis occurs mostly in middle-aged and elderly patients who are not breastfeeding, but nowadays it also occurs in some young women, some of whom have not yet had children, which means that there is a trend toward younger patients with plasmacytoid mastitis. Plasmacytoid mastitis is a common and difficult to treat disease in breast surgery. The main causes are congenital nipple invagination, poor milk flow during breastfeeding or short breastfeeding periods, and forced withdrawal of milk, which causes milk to accumulate in the milk ducts; some patients are obese and have more local oil, which blocks the milk ducts. Clinically, both plasmacytoid mastitis and breast cancer can present as painless breast lumps with hard texture, unclear borders, uneven surface and poor mobility; it is also difficult to distinguish them on imaging. In such cases, the two are differentiated under the microscope using a breast mass aspiration biopsy, in which a portion of the mass is removed for pathological testing through a minimally invasive procedure. Plasmacytoid mastitis can be seen microscopically with most of the plasma cells infiltrating and present at the lesion site, and breast cancer microscopically with changes of cancer cells. Therefore, the effective diagnosis is pathological examination. The clinical manifestations are acute and chronic. Acute mastitis lesions are locally red, swollen, hot and painful, with breast abscesses. Chronic mastitis has no obvious local skin flushing, mostly lumps, the pus cavity collapses on its own, many curved sinus tracts are formed under the pus cavity, the breast appears to be this or that, the wound does not heal for a long time, and a few patients may also have plasma, bloody or purulent nipple overflow. This disease is prone to recurrence, long-term wound healing and breast disfigurement, and belongs to the category of “acne vulgaris” in Chinese medicine.