Allo-HSCT appears to be an efficient tool to cure patients with MDS and AML secondary to MDS. At present, the best conditioning regimen for MDS and AML secondary to MDS undergoing allo-HSCT remains in discussion. BUCY conditioning regimen is the standard myeloablative regimen for MDS and AML secondary to MDS undergoing allo-HSCT. However, it appears to have higher relapse rate. To reduce the relapse rate, decitabine is added in the conditioning regimen. In this prospective randomized controlled study, the safety and efficacy of G-CSF + Decitabine + BUCY and BUCY myeloablative conditioning regimens in RAEB-1, REAB-2 and AML secondary to MDS undergoing allo-HSCT are evaluated.