Venetoclax Combining With Fludarabine and Melphalan as Conditioning Regimen for Allo-HSCT

Inclusion Criteria: 1. Age older than 50 years 2. Patients diagnosed with acute myeloid leukemia, acute lymphoblastic leukemia, myelodysplastic syndrome according to WHO diagnostic criteria. 3. Patients who achieved complete remission before transplantation. 4. Patients who have alternative donors and plane to accept allogeneic hematopoietic stem cell transplantation treatment. 5. ECOG body status score 0-2. 6. Good organ function level: ANC (neutrophil absolute value \>=1.0x10\^9/L; PLT \>=30x10\^9/L; HB \>=80g/L; Tibil \<=1.5 ULN; ALT / AST \<=2.5 ULN; bun / Cr \<=1.5 ULN; LVEF \>=50%). 7. Patients who voluntarily participate in the clinical trial, understand the research procedure and can sign the informed consent in writing. Exclusion Criteria: 1. Patients who didn't achieved complete remission before transplantation (bone marrow smear: proportion of primordial cells \>=5%) or any extramedullary recurrence. 2. Patients who were previously known to be resistant to venetoclax. 3. In patients with severe pulmonary insufficiency (obstructive and / or restrictive ventilation disorders), the researchers evaluated the patients who could not tolerate the super pretreatment scheme. 4. Patients with severe liver function impairment and liver function indexes (alt, TBIL) more than 3 ULN were evaluated as intolerant of super pretreatment. 5. In patients with severe renal insufficiency, the renal function index (CR) is more than 2 times of the upper limit of the normal value (ULN), or the 24-hour creatinine clearance rate (CR) is less than 50ml / min, the researchers evaluated that they could not tolerate the super pretreatment scheme. 6. In patients with severe active infection, the researchers evaluated that they could not tolerate the pretreatment. 7. Patients who had allergic reactions or serious adverse reactions in the previous use of pretreatment related drugs could not be included in the study. 8. Other reasons why the researchers could not be selected.