There are many reasons for sudden and severe anemia, and the common ones are bleeding, acute hemolysis, acute hematopoietic dysfunction and so on. 1. Bleeding: when certain diseases such as liver cirrhosis and peptic ulcer lead to hemorrhage in the digestive system, the number of red blood cells of the patient will be drastically reduced, which triggers severe anemia or even hemorrhagic shock. 2. Acute hemolysis: when the patient’s anemia input blood type does not match, a large number of red blood cells will be destroyed, and hemoglobin will drop sharply in a short period of time. 3. Acute hematopoietic dysfunction such as acute leukemia, the normal hematopoietic function of bone marrow is inhibited, causing severe anemia. There may also be other reasons for sudden severe anemia, such as acute renal failure, major blood loss after trauma or surgery, tumors and so on. When anemia suddenly worsens, it is recommended to consult a doctor in time, complete relevant examinations, and carry out targeted treatment and therapy after clarifying the cause of the disease.