The goal of this phase 2 trial is to test the safety and efficacy of Venetoclax combined with Azacytidine as Maintenance Therapy post Hematopoietic Stem Cell Transplantation in Myelodysplastic Syndromes and Acute Myeloid Leukemia.

The investigators will evaluate the efficacy and safety of venetoclax combined with azacytidine as maintenance therapy post hematopoietic stem cell transplantation in myelodysplastic syndromes and acute myeloid leukemia. overall survival (OS), cumulative 1-year relapse- and progression-free survival (RPFS), time to relapse, relapse rate, incidence of acute and chronic GVHD, time to treatment discontinuation will be counted.

Inclusion Criteria:

• The patient should, in the investigator's opinion, be able to meet all clinical trial requirements.
• The patient is willing and able to adhere to the study visit schedule and other protocol requirements.
• Patients aged ≥18 years with a diagnosis of MDS or AML according to World Health Organization criteria (WHO) who had undergone alloSCT with myeloablative or reduced-intensity conditioning regimens were eligible. Related and unrelated donors were permitted.
• Patients should be in morphologic complete remission (CR; ie, ≤5% bone marrow blasts) with absolute neutrophil counts ≥1.0×10^9 /L and platelets ≥ 75×10^ 9 /L before treatment.

Exclusion Criteria:

• Active or uncontrolled infections requiring systemic treatment within 14 days before enrollment.
• Any instability of systemic disease, including but not limited to severe cardiac, liver, kidney, or metabolic disease need therapy.
• Pregnant or lactating women.