Breast pain on the left side of the breast is mainly seen in three cases, as follows: 1) cystic hyperplasia of the breast, seen in women aged 30-50 years old, the typical clinical manifestation of breast swelling and pain, accompanied by breast hyperplasia nodule formation, the pain is mostly distending or stabbing, and aggravated before menstruation, and can be relieved after menstruation.2) Acute mastitis, commonly seen in breastfeeding women, is an acute suppurative infectious disease of breast tissues, with the combination of breast milk stagnation. It is an acute purulent infection of breast tissue, combined with breast milk stagnation. In the early stage of infection, breast swelling and pain may appear, and when the infection is aggravated, the pain will be aggravated progressively, combined with fluctuating lumps.3. Breast cancer is common in elderly women, and it has no obvious manifestation in the early stage of the disease, but when the cancer invades into subcutaneous tissues, it may appear as breast pain, and there is the orange peel sign or the dimple sign. Therefore, when there is pain on the left side of the breast, breast color ultrasound or fine needle aspiration biopsy should be performed in time to make a clear diagnosis, and reasonable treatment should be carried out under the guidance of professional doctors.