Sufficient arterial pressure at the distal extremity of the limb determines the good reflux of the veins, and good venous reflux can ensure the filling of superficial veins. The tension of the tourniquet in the limbs, the pressure of the blood vessels is different, so the effect on the distal limb blood flow is different. The normal arterial pressure of the human upper limb is 13. 3~16.0kpa (100~120MMHG), peripheral elbow anterior venous pressure was 10.0cm $literal. Theoretically speaking, when the tourniquet pressure reaches 13.3-16.0kpa, it can block the arterial blood flow of the upper limb, and 10cm $literal pressure can block the venous reflux of the upper limb. However, it is found that the above two kinds of pressure can not completely block the blood flow of the artery and vein in the sex bed practice. The pressure to completely block arterial blood flow in the upper extremity is 40kPa (300mmHg). Venipuncture requires a good filling of superficial veins. Only by guaranteeing good arterial pressure at the distal extremity of the upper limb, and completely blocking the return of superficial vein, the venous filling of the distal limb can reach the best state. Tourniquet pressure is too high, while blocking superficial veins, the arteries are blocked and the arterial blood flow in the distal extremities is reduced, leading to poor venous filling. When the tourniquet pressure is too low, although the arterial blood flow in the distal extremity is good, the venous reflux is not completely interrupted and the venous filling at the distal extremity is not ideal. Through the experiment, we found that when the tourniquet pressure was 10.7~16.0kpa, the venous filling of the distal limb reached its optimal state.