Blood viscosity is the resistance formed by the friction between two parallel layers of fluid flowing close together and displaced from each other as the blood flows through the blood vessels. The viscosity of whole blood is a combination of the denaturation and aggregation capacity of red blood cells, and the rheological properties of platelets and white blood cells. Blood viscosity is also affected by physicochemical factors such as temperature, pH, plasma osmolarity, vascular factors, and sometimes the perfusion volume of blood flow, so there are two types of viscosity: high-shear and low-shear. The high-shear viscosity reflects the deformability of red blood cells, and the low deformability makes the blood viscosity higher. The low-shear viscosity reflects the cohesive ability of red blood cells, and the blood viscosity increases when red blood cells coalesce with each other. In many diseases, the viscosity of whole blood increases only at low shear, which is related to the slow blood flow caused by low shear and the easy coagulation of red blood cells. The measurement of whole blood viscosity can provide a reference for the diagnosis, treatment and prevention of various clinical diseases, especially the pre-thrombotic state and thrombotic diseases.