Abstract: Immune deficiency of the host caused by allogeneic hematopoietic stem cell transplantation (allo-HSCT) is the initial factor of reactivation of latent human cytomegalovirus (HCMV). The risk factors of reactivation of HCMV in allo-HSCT recipients consist of the serological status of HCMV in donors and recipients, the matching degree of human leukocyte antigen (HLA) and pretreatment patterns, etc. The reactivation of HCMV is associated with the expression of a series of viral cleavage and proliferation proteins induced by the overexpression of major immediate early promoter/enhancer (MIEP) in the viral genome. In this article, the risk factors of reactivation of HCMV after allo-HSCT, the molecular changes related to maintaining latent infection of HCMV, the key role of MIEP overexpression in reactivation of HCMV, and the molecular pathways involved in reactivation of HCMV after allo-HSCT were reviewed and the major molecular events of reactivation of HCMV after allo-HSCT were elucidated, aiming to provide reference for the prevention and treatment of cytomegaloviral disease (CMVD) after allo-HSCT.