Principles of clinical blood transfusion include indications for blood transfusion, pre-transfusion preparation, route of blood transfusion, speed of blood transfusion, etc. Precautions for blood transfusion include pre-transfusion, transfusion and post-transfusion precautions. 1. Clinical blood transfusion principles (1) Indications for blood transfusion: including massive blood loss, correction of anemia, coagulation abnormality, supplementation of plasma proteins and improvement of body resistance. (2) Pre-transfusion preparation: carry out transfusion compatibility tests such as ABO blood group (ortho-typing and anti-typing), RhD blood group, antibody screening and cross-matching. (3) Transfusion route: the most common and convenient route is intravenous blood transfusion. (4) Transfusion speed: 5 ml per minute for adults, 1 ml per minute for elderly or heart disease patients, and 10 drops per minute for children; the input speed can be increased appropriately in case of hemorrhage. 2. Precautions for blood transfusion (1) Before blood transfusion: must carefully comply with the information of blood supply and acceptance, the result of blood transfusion compatibility test, and check whether the blood bag is broken or not, and whether there is any abnormality in the nature of blood. In addition to saline injection for dilution, it is prohibited to add other drugs. (2) During blood transfusion: vital signs and changes in urine should be closely examined, and problems should be dealt with promptly. (3) After blood transfusion: need to stay for a certain period of time to avoid delayed transfusion reaction, blood should be retained for at least 1 day, and blood samples should be properly retained for at least 7 days.