Purple bruising and tendon pain in the arm after blood donation is usually caused by subcutaneous bruising after blood donation, which is often related to improper pressure after blood donation or operational errors of the blood collector. Tendon pain, which is actually pain in the blood vessels at the puncture site, is caused by the fact that thicker and better conditioned blood vessels are often chosen for blood donation, and the blood collection needles are thicker, which may cause damage to the blood vessel walls during blood collection and cause pain. A small number of patients have purple bruised tendon pain in the arm after blood donation, which may also be caused by abnormal blood clotting function. Purple bruises and pain in the arm after blood donation may not be treated temporarily if they are not accompanied by other uncomfortable symptoms, and will usually resolve on their own after a period of time. Blood donors can use hot compresses with towels 24 hours after blood donation to promote local blood circulation and help absorption of subcutaneous bruises. In addition, avoid strenuous exercise for 24 hours after blood donation and try not to use the donor’s arm to lift heavy objects for 1-2 days. If the purple bruising and tendon pain in the arm does not subside for a long period of time or even worsens, the donor is advised to go to a regular hospital for a coagulation test to clarify the cause for early treatment. If thrombocytopenia occurs, standardized treatment under the guidance of a hematologist is required. Glucocorticoids, gammaglobulin and other drugs can be used for treatment, and immunosuppressive drugs can be used if necessary.