Severe anemia of 50g/L requires blood transfusion, because in severe anemia the patient’s circulatory and respiratory systems are significantly damaged, so blood transfusion should be given in time to correct the anemic state and avoid anemic heart disease, heart failure and respiratory failure. In general, for patients without heart disease, the hemoglobin level should be increased to at least 60 g/L, while for patients with heart disease, the hemoglobin level should be increased to at least 70 g/L. Otherwise, for patients with heart disease, anemia can easily induce angina pectoris or even myocardial infarction. Long-term severe anemia, i.e., when the hemoglobin is below 60g/L, may cause anemic heart disease, heart failure and a series of manifestations of multi-system damage such as respiratory failure, abnormal nervous system, decreased appetite and poor nutrition, so the patient should be given timely blood transfusion treatment. Blood transfusions are given for rapid correction of anemia on the one hand and to provide opportunity and time for diagnosis and treatment of the cause of anemia on the other hand. Anemia is divided into 4 major categories according to different etiologies, anemia caused by abnormal hematopoietic cells such as acute leukemia, aplastic anemia, and myelodysplastic syndrome. It also includes anemia caused by insufficient hematopoietic material, such as iron deficiency anemia and megaloblastic anemia, along with hemolytic anemia and blood loss anemia.