Thick blood means increased viscosity of the blood. When the blood viscosity increases, the blood flow is slowed down, which can easily cause ischemia and hypoxia in vital organs, and also induce various thrombotic and embolic diseases. Blood thickening is a general term for a group of diseases, including a variety of diseases, increased blood viscosity caused by an increase in the number of various blood cells, as well as changes in various solute components in the plasma can also cause blood viscosity. In clinical practice, conditions such as true erythrocytosis, primary thrombocytosis, and hyperleukocytic leukemia can cause thickening of the blood due to an increase in the number of blood cells. Also, alterations in the solute composition of the plasma such as hyperglycemia, hyperlipidemia, and hyperfibrinogenemia can cause blood viscosity. In addition, malignant tumors of the hematological system, such as multiple myeloma, are also prone to thick blood due to the production of large amounts of monoclonal type immunoglobulins. Therefore, thick blood is a group of diseases with multiple causes, and different treatments need to be given depending on the cause.