Purple skin of the mouth, also known as cyanosis, often indicates hypoxia. Cyanosis is a clinical manifestation of increased reduced hemoglobin in the blood, resulting in cyanotic changes in the skin and mucosal layer, and is often associated with hematologic diseases, circulatory diseases, respiratory diseases, etc. Cyanosis can also occur in the case of certain toxic poisonings, such as carbon monoxide poisoning. For the appearance of purple skin on the mouth, blood routine, blood gas analysis, arterial oxygen saturation measurement, chest CT, electrocardiogram, pulmonary function test, bronchoscopy, etc. are needed to further clarify its related etiology and carry out the next step of treatment.