Endothelial damage and bleeding after endothelial plaque rupture are the main causes of red thrombus formation, which can further activate the coagulation system of the body and slow down the blood flow by forming white thrombus where platelets gather in the area of rapid blood flow and constitute the head of mixed thrombus. Platelets, fibrin, and neutrophils aggregate to form a mixed thrombus, which further enlarges and obstructs the lumen of the blood vessel, slowing or even stopping the local blood flow, resulting in platelets depositing there and forming a red thrombus with fibrinogen and aggregated red blood cells, thus forming the tail of the mixed thrombus. Under direct visualization, the red thrombus resembles a blood clot and is dark red in color, while microscopically more red blood cells and fibrin network are seen to be aggregated. The fresh red thrombus is elastic and has a moist surface, while the old red thrombus loses elasticity due to massive water loss and becomes dry, and easily breaks off or falls off to form an embolus.