Inclusion Criteria:  Aged ≥18 and ≤80 years old.  CD20-positive relapsed/refractory DLBCL (≥ 2 prior lines of therapy )  Recurrence: Relapse occurred more than 6 months after the end of treatment. At least one regimen contains Rituximab.  Refractory: Relapse within 6 months after the end of treatment or fail to reach PR after 2 treatment cycles and fail to reach CR after 4 treatment cycles. At least one regimen contains Rituximab.  Recurrence after second-line treatment sequential autologous hematopoietic stem cell transplantation.  Eastern Cooperative Oncology Group (ECOG) performance status ≤ 2 More than 3 months life expectancy. Those who agree to provide archived tumor tissue samples or fresh tissue samples Biopsy confirmed CD20-positive after the last treatment. Recurrence more than 1 year needs to undergo tissue biopsy to confirm the pathological diagnosis. Adequate cardiac function (LVEF≥50%).  At least one measurable lesion:  For intranodal lesions, the long diameter should be >1.5cm; for extranodal lesions, the long diameter should be >1.0cm.  Neutrophil count (NEUT) ≥1.5×109/L and platelet count (PLT) ≥75*109/L and hemoglobin ≥75g/L, total bilirubin level (TBIL) ≤1.5×upper limit of normal (ULN), aspartic acid Aminotransferase (AST), alanine aminotransferase (ALT)≤2.5×ULN, creatinine level (Cr)≤1.5×ULN. Patients with liver metastases (TBIL≤3×ULN, ALT/AST≤5×ULN). Signed an informed consent form which was approved by the institutional review board of the respective medical center.  Exclusion Criteria:  Primary mediastinal (thymic) large B-cell lymphoma. Transformed lymphoma. DLBCL invaded by special parts, such as central nervous system (CNS), testis, breast, ovary, etc. High-grade B-cell lymphoma with MYC, BCL2 and/or BCL6 rearrangement. History of other malignancies (except for basal/squamous cell carcinoma of the skin, in-situ carcinoma of the cervix or breast, superficial bladder cancer, lung carcinoma in situ that have been cured for more than 5 years). Infection with human immunodeficiency virus (HIV) or chronic infection with hepatitis B virus (HBV-DNA titer higher than 2000 IU/mL or 1000 copies) or hepatitis C virus (HCV). Treponema pallidum (TP) antibody positive. Received systemic anticancer therapy include radiation, targeted therapy, or any other anticancer therapy within 3weeks or 5 half-lives before the first administration. Participation in another clinical trial in the past 4 months. Received allogeneic stem cell transplantation. Previous treatment with anti-PD-1, anti-PD-L1, anti-PD-L2, anti-CD137 or anti-CTLA-4 antibody or other medications that stimulates or collaboratively inhibits T cell receptors or CAR-T. Usage of immunosuppressive agents, systemic or local hormone therapy within 14 days before the first administration to achieve the purpose of immunosuppression (the daily dose is equivalent to systemic corticosteroids with prednisone> 10 mg). Vaccination with an attenuated live vaccine within 4 weeks before the first administration. Suffered from interstitial pneumonia in the past 6 months. Severe cardiovascular disease (New York Heart Association functional class III or IV, myocardial infarction or unstable arrhythmia or unstable angina in the last 6 months, severe cardiac insufficiency, rogressive multifocal leukoencephalopathy) Uncontrolled hypertension (SBP≥180mmHg and/or DBP≥100mmHg). Active autoimmune diseases, including but not limited to systemic lupus erythematosus, ankylosing spondylitis, etc. Severe mental illness. Known hypersensitivity to any of the study drugs or its ingredients. Pregnant or lactating women. The researcher believes that it is not suitable for enrollment.