Dermatomyositis is a group of autoimmune diseases that mainly involve the skin and muscles, usually involving both skin and muscle causing lesions, while those with muscle damage only are called polymyositis. The cause of the disease is still unclear and may be closely related to genetic factors, bacterial and viral infections, malignancies and drugs. The pathogenesis may be related to autoimmunity. Patients may have anti-Jo-1 antibodies, anti-PL-7 antibodies, anti-myocardin antibodies and other antibodies. There is CD4+ T-cell infiltration around the vessels of the diseased muscles and deposition of IgG, IgM and C3 in the basement membrane of the vessel wall. The muscle groups most frequently invaded by muscle damage are the proximal muscle groups of the extremities, the scapular girdle muscle group, and the neck and throat muscle groups. Clinical manifestations such as hand raising, head lifting, stair climbing, squatting, dysphagia and hoarseness may be observed. In severe cases, the disease may involve the respiratory muscles and cause respiratory distress, while the heart muscle may be involved with palpitations, arrhythmia, and heart failure in advanced stages of the disease. It may also be accompanied by irregular fever, unexplained weight loss, and swelling and pain in the joints. The clinical manifestations vary depending on the muscle groups involved, but most cases are complex and serious, so it is very important for patients with suspected polymyositis to seek early diagnosis and treatment from a rheumatologist.