Mannitol infusion will hurt the reason may be related to mannitol infusion too fast, high concentration of mannitol particles, mannitol extravasation and other factors. 1. Mannitol infusion is too fast: Mannitol infusion is too fast, which can make the mannitol drug enter into the blood vessel quickly in a short period of time, and the concentration of mannitol at the infusion place is high, due to the change of the concentration gradient, the water in the endothelial cells migrates out of the cells, the cells are dehydrated and denatured, and the blood vessels are hardened. At this time there will be symptoms of redness, swelling and pain of the skin at the infusion site. 2. High concentration of mannitol particles: Due to the high concentration of mannitol drug, mannitol particles agglutinate, causing red blood cell adhesion and aggregation blockage when passing through small blood vessels. Tissue hypoxia after blockage of blood vessels supplying blood, resulting in edema and inflammation, and pain. 3. Mannitol extravasation: When mannitol destroys the blood vessels and extravasates into the subcutaneous tissue, it is not easily absorbed by the tissue, resulting in a contrast in the osmotic gradient, prompting more fluid to leak out from the blood vessels into the tissues, which can damage the tissues and cause pain. Pain during mannitol infusion requires prompt communication with the attending physician for timely prevention and treatment.