This is a single-arm, early safety and feasibility exploratory study to evaluate the efficacy and safety of MegaLT Injection in promoting platelet recovery after umbilical cord blood transplantation. Adults with malignant hematologic diseases who undergo umbilical cord blood transplantation will receive a single intravenous infusion of MegaLT Injection on day 14 after transplantation. Two dose levels will be evaluated sequentially, and platelet recovery, platelet transfusion requirements, and safety outcomes will be assessed.