Bruising of the arm after blood donation is due to the fact that the location of the blood vessel that was punctured after blood donation was not pressed tightly, resulting in localized hemorrhage on the vessel wall, which penetrated into the subcutaneous tissue and was formed. In the early stage, it is manifested in the place where the blood was drawn and there is swelling. Because of the blood inside, until slowly the blood is absorbed by the body tissue after the formation of a large bruise under the skin. Therefore, after the blood is drawn, the place where the blood is drawn should be pressed very tightly, and at the same time, do not rub back and forth when pressing. In the later stages of bruising, use a warm towel, or hot things to the surface of the hot compresses, to accelerate the circulation of blood around, so that the bruise will fade faster.