This study is conducted in a prospective, single-center clinical design and is divided into two stages: dose escalation and dose extension. Patients meeting the diagnostic criteria of secondary poor graft function are selected as the study objects. The safety data of umbilical cord blood mononuclear cells in the treatment of secondary poor graft function are obtained through dose escalation stage, and then one dose is selected for dose extension stage to explore the efficacy of umbilical cord blood mononuclear cells in treating secondary poor graft function.

Inclusion Criteria:

1. Age≥18 years old; gender is not limited.
2. Meet the diagnostic criteria for secondary poor graft function: After 28 days of transplantation, patients who had achieved hematopoietic reestablishment (ANC ≥ 0.5×10^9/L for 3 consecutive days without G-CSF application, PLT ≥ 20×10^9/L for 7 consecutive days without platelet infusion, Hb ≥ 80g/L for 2 consecutive weeks without red blood cell infusion) again developed two or three line cytopenia lasting more than 2 weeks. Bone marrow examination revealed low myelodysplasia, remission of primary disease, complete donor chimeric cells, and no severe graft-versus-host disease (GVHD) or disease recurrence.
3. Eastern Cooperative Oncology Group (ECOG) physical status score ≤ 2 points.
4. Subjects sign informed consent.

Exclusion Criteria:

1. Serious infection not controlled.
2. Active bleeding.
3. Patients with cardiac insufficiency (ejection fraction<50%), or suffering from serious heart disease, including myocardial infarction, cardiac insufficiency, etc.
4. Patients with hepatic and renal insufficiency (total bilirubin>35µmol/L, ALT and AST>2 times of the upper limit of normal; serum creatinine>130µmol/L).
5. Pregnant or lactating women.
6. Concurrent malignant tumors of other organs.
7. Failure to understand or follow the research protocol.
8. Patients participating in other clinical investigations.
9. Other conditions that the investigators consider inappropriate to participate in the study.