• Progression-Free Survival (PFS) [ Time Frame: up to 96 weeks ]
  PFS defined as the time from randomization until the first documented progressive disease (PD) or death from any cause.
• Duration of Response (DOR) [ Time Frame: up to 96 weeks ]
  DOR defined as time from earliest date of disease response to earliest date of disease progression based on radiographic assessment.
• Disease Control Rate (DCR) [ Time Frame: up to 24 months ]
  Percentage of subjects achieving complete response (CR) and partial response (PR) and stable disease (SD).
• Time to Response (TTR) [ Time Frame: up to 24 months ]
  TTR defined as time from the first dose to the first assessment of PR or CR.
• Overall Survival (OS) [ Time Frame: up to 24 months ]
  OS defined as the time from randomization to death from any cause. Subjects who do not die at the end of the extended follow-up period, or were lost to follow-up during the study, were censored at the last date they were known to be alive.

Inclusion Criteria:

• 1. Histologically confirmed relapsed or refractory primary mediastinal large B-cell lymphoma.

  2. 18 and 75 years; Eastern Cooperative Oncology Group (ECOG) performance status score of 0 or 1; Life expectancy > 3 months.

  3. At least one measurable lesion. 4. Left ventricular ejection fraction (LVEF) measured by the cardiac echocardiography ≥ 50%.

  5. Screening laboratory values must meet the following criteria:hemoglobin ≥ 80 g/L; neutrophils ≥ 1.5*10^9/L; platelets ≥ 100 x 10^9/ L.

  6. Understood and signed an informed consent form.

Exclusion Criteria:

• 1. Hypersensitivity to recombinant humanized anti-PD-1 monoclonal Abm or its components.

  2. Prior therapy with an anti-programmed cell death (PD)-1, anti-PD-L1, anti-PD-L2, or anti-cytotoxic T-lymphocyte-associated antigen-4 (CTLA-4) antibody ,or any other antibody or drug specifically targeting T-cell co-stimulation or checkpoint pathways.

  3. Has received chemotherapy, surgery, radiotherapy, the last treatment from the first dose less than 4 weeks, or oral targeted drugs for less than 5 half-lives, or oral fluorouracil pyridine drugs for less than 14 days, mitomycin C and nitrosourea for less than 6 weeks.

  4. Has received emergency cytoreductive surgery to control tumors. 5. Has received allogeneic hematopoietic stem cell transplantation within the last 5 years.

  6. Has adverse events caused by previous therapy except alopecia that did not recover to ≤grade 1.

  7. Has diagnosed and/or treated additional malignancy within 5 years prior to randomization. Exceptions include basal cell skin cancer, squamous cell carcinoma of skin, melanoma skin and cancer carcinoma in situ of the cervix.

  8. Has definite central nervous system (CNS) infiltration of lymphoma, including brain parenchyma, meningeal invasion or spinal cord compression.

  9. Has any active autoimmune disease or a history of autoimmune disease. 10. Has serious or uncontrolled diseases such as history of chronic heart failure.

  11. Has any active autoimmune disease or a history of autoimmune disease. 12. Has received blood transfusion, erythropoietin granulocyte colony stimulating factor(G -CSF)，or Granulocyte macrophage colony stimulating factor(GM-CSF) within 4 weeks before the first dose.

  13. Has vaccinated with vaccines or attenuated vaccines within 4 weeks before the first dose.

  14. Has received surgery, or unhealed wounds within 4 weeks before the first dose.

  15. Has Hepatic, renal, blood coagulation dysfunction. 16. Has interstitial lung disease or non-infectious pneumonia and present residual lesions.

  17. Has received systemic treatment for active infection before the first dose.

  18. Has active or latent tuberculosis. 19. Hepatitis B virus surface antigen (HBsAg) positive, and hepatitis B virus DNA copy number > upper limit of normal.

  20. Human immunodeficiency virus antibody positive , hepatitis C antibody (HCV-Ab) and hepatitis C virus DNA copy number > upper limit of normal.

  21. Breastfeeding or pregnant women. 22. According to the judgement of the researchers, there are other factors that subjects are not suitable for the study.