This study was designed as a prospective, multicenter, open-label, randomized controlled trial. Eligible participants were patients aged 15-60 years with high-risk or relapsed/refractory acute myeloid leukemia (AML), acute lymphoblastic leukemia (ALL), or myelodysplastic neoplasms (MDS), diagnosed based on bone marrow morphology, immunophenotyping, genetic testing, and treatment response assessment. The experimental group received SHR2554 combined with azacitidine as an overlapped sequential combination with the TBF conditioning regimen, whereas the control group received the mBuCy conditioning regimen, both followed by allogeneic hematopoietic stem cell transplantation (allo-HSCT). The primary endpoint was the 2-year cumulative incidence of relapse (CIR) after allo-HSCT. Secondary endpoints included 2-year overall survival (OS), 2-year disease-free survival (DFS), transplant-related mortality (TRM), the incidence of acute and chronic graft-versus-host disease (GVHD), and safety profiles.