The causes of severe anemia include the following: 1. anemia caused by the lack of hematopoietic raw materials needed to produce red blood cells, commonly seen in iron deficiency anemia and megaloblastic anemia. 2. hemolytic anemia caused by increased destruction of red blood cells, commonly seen in thalassemia, sericosis, hereditary spherocytosis, paroxysmal sleep hemoglobinuria, autoimmune hemolytic anemia. 3. Anemia caused by damage, such as myelodysplastic syndrome, myelofibrosis, aplastic anemia. 4, Malignant clonal diseases of hematopoietic stem cells, such as acute leukemia, chronic leukemia. 5, Blood loss, such as post-traumatic hemorrhage, hemorrhoids, chronic blood loss caused by excessive menstrual flow.