The goal of this phase I/II clinical trial is to test in relapsed or refractory acute lymphoblastic leukemia (R/R ALL) patients undergoing allogeneic hemopoietic stem-cell transplantation (allo-HSCT). The main question it aims to answer is:

• The efficacy and safety of blinatumomab maintenance therapy in reducing the recurrence rate a in R/R ALL patients after allo-HSCT. Participants will take intravenous blinatumomab after allo-HSCT. The dose of one course was as follows: day 1-2: 8ug/day, continuous intravenous drip for 24 hours, day 3-7: 16ug/day, continuous intravenous drip for 24 hours. Treatment with blinatumomab was initiated within 60 to 90 days after transplantation and was administered bimonthly until 1 year after transplantation. Dexamethasone 20mg was administered 1 hour before administration on days 1 and 3 to prevent adverse events.

Inclusion Criteria:

1. B-ALL patients with history of relapse, or MRD positive in the last bone marrow examination before allo-HSCT;

2. Age ≥16 years old and ≤ 65 years old when signing informed consent Form (ICF);

3. KPS > 60 or ECOG 0-2;

4. The expected survival time is more than 3 months;

5. Complete remission (CR) after allo-HSCT with either myeloablative or non-myeloablative conditioning regimen determined by the investigator;

6. Reach the standard of hematopoietic reconstitution (neutrophil count

   ≥ 0.5×10^9/L for 3 consecutive days without G-CSF application, platelet count ≥ 20×10^9/L for 7 consecutive days without platelet transfusion, Hb ≥ 80 g /L without red blood cell transfusion); and neutrophil count ≥ 1.5×10^9/L, platelet count ≥ 50×10^9/L within 45 days after transplantation;

7. No central nervous system involvement or clinical symptoms after transplantation;

8. Those who have no serious functional damage to important organs of the body;

9. Fully understand and be informed of this study and sign the ICF; willing to follow and have the ability to complete all test procedures;

10. Females of childbearing age must afford a serum pregnancy test within 7 days before the first dose, and the result should be negative; female participants and their partners should agree to use effective contraception from signing the ICF until 6 months after the last dose.

Exclusion Criteria:

1. Serious basic diseases of important organs: such as myocardial infarction, chronic cardiac insufficiency, decompensated hepatic insufficiency, renal function, gastrointestinal insufficiency, etc.;
2. Uncontrolled active infection (including bacterial, fungal, or viral infection), and drug treatment is ineffective;
3. Participating in other clinical studies, or planning to start treatment in this study and less than 4 weeks before the end of treatment in the previous clinical study;
4. Poor graft function (PGF) occurred after allo-HSCT;
5. Combined with other malignant tumors and require treatment;
6. Active GVHD;
7. Have a history of allergy to Chidamide;
8. Pregnant or lactating females;
9. Patients with known history of human immunodeficiency virus (HIV) virus infection and/or acquired immunodeficiency syndrome;
10. Patients with active chronic hepatitis B or active hepatitis C;
11. History of prolonged QT syndrome;
12. Patients considered by other researchers to be unsuitable for this study