A nevus is a pigmented nevus, a benign tumor formed by the local aggregation of nevus cells. The main harm is to affect the aesthetics and thus may have a psychological impact on the patient. At the same time, nevus also has the possibility of malignant transformation, but the chance of malignant transformation is low overall. First of all, nevus nigricans usually grow on the face, chest, lower limbs and other parts of the body, with black color and long hair, especially facial nevus nigricans can seriously affect the image of patients, and can seriously affect social life and sometimes cause great psychological problems to patients. Secondly, although nevus is a benign tumor, there is also a risk of malignant transformation. When patients with nevus are in high radiation places or repeatedly rubbed locally for a long time, it may cause proliferation and spread of nevus, as well as local ulceration, redness and swelling and other cancerous phenomena. Most of the nevi belong to mixed nevi, which have certain potential for malignant transformation. The probability of congenital giant nevus becoming malignant is about 4%, and it usually has a high degree of malignancy after malignant transformation. Patients should pay more attention to the changes of the disease. If the hairs on the mole fall off by themselves, and if there is redness, swelling, hard nodules or pus around the mole, the possibility of malignant transformation should be considered, and timely medical consultation is needed.