Allogeneic haematopoietic stem cell transplantation (allo-HSCT) remains one of the currently available curative therapies for acute leukemia (AL). Leukemia relapse is one of the mainly causes of transplant failure. We reported previously that patients with high-risk molecular biomarkers who still have detectable minimal residual disease(MRD) pre-HSCT were at very high risk of relapse, with cumulative relapse rate of 50-80%. Decitabine has been demonstrated efficacy in the treatment of patients with recurrent or refractory leukemia and myelodysplastiv syndrome. It was reported that the combination of decitabine, with busufan and cyclophosphamide as a preparative regimen for allo-HSCT using HLA-matching donors was safe and effective. In this prospective, single-arm clinical trial, we aimed to examine the efficacy of combining decitabine with modified busulfan and cyclophosphamide (mBU/CY) as a preparative regimen for allo-HSCT in patients with very high-risk AL and detectable MRD pre-HSCT.