Blood viscosity is related to the tangency of blood flow, and the normal range of blood viscosity for different tangencies is between 5.61±0.85 mPa-s and 10.4±1.0 mPa-s. Blood viscosity is the frictional resistance between two adjacent parallel fluid layers in blood flow. Blood flow cutoff is the ratio of the difference between the blood flow velocity in two adjacent layers and the thickness of the fluid layer in the case of laminar flow. Whole blood is a non-homogeneous fluid and its viscosity increases with decreasing shear rate. The normal value of blood viscosity is related to the shear rate of blood flow: At a cut rate of 115 (S-1), the reference value for blood viscosity is 5.61 ± 0.85 mPa-s; At a cut rate of 46 (S-1), the reference value for blood viscosity is 7.3±1.1 mPa-s; At a cut rate of 11.5 (S-1), the blood viscosity reference value was 10.4 ± 1.0 mPa-s. The measurement of blood viscosity can provide a certain reference basis for the diagnosis and treatment of many diseases and prevention, especially significant in the diagnosis and treatment of pre-thrombotic state and thrombotic diseases. Therefore, when the blood viscosity test result is out of the normal range, attention should be paid to it, and intervention should be carried out after the etiology is clarified as early as possible.