Cyanosis refers to the increase of reduced hemoglobin content or abnormal hemoglobin derivatives in the blood, resulting in cyanosis of the skin and mucous membranes. It is often obvious in the parts of mouth and lips, nail bed and cheek, because in these parts, capillaries are abundant, skin is thin and pigmentation is less, so it is easy to observe. The common causes of cyanosis are as follows: 1. respiratory diseases: cyanosis can occur in severe hypoxemia caused by various pulmonary diseases; 2. heart diseases: left heart failure due to various causes, congenital cyanotic heart disease, etc.; 3. peripheral circulation disorders: shock, pericardial effusion, constrictive pericarditis, peripheral venous lesions, etc.; 4. increased abnormal hemoglobin derivatives: methemoglobinemia, Sulfated hemoglobinemia, etc., all of which can show cyanosis symptoms. When cyanosis symptoms appear, it is important to go to the hospital for examination, which requires blood routine, blood gas analysis, electrocardiogram and chest X-ray, echocardiography, cardiac catheterization and cardiovascular angiography if necessary. The diagnosis should be clarified as soon as possible and targeted treatment should be given as early as possible to avoid delaying the condition.