Inclusion Criteria:
1. The initial diagnosis was primary diffuse large B-cell lymphoma of the central nervous system confirmed by histopathology;
2. Prior to first-line treatment based on methotrexate (with or without radiotherapy), tumor recurrence was confirmed by MRI;
3. Measurable focus in MRI (\>10x10mm);
4. Aged \> 18 years;
5. Life expectancy of at least 12 weeks;
6. The patient has a Karnofsky performance status of at least 50%;
7. Main organs function normally, without serious blood, heart, lung, liver, kidney and immune deficiency diseases. Specific assay indicators requirements： White blood cells\>3.0×10\^9/L;platelet\>80×10\^9/L;hemoglobin\>10g/dL;serum bilirubin ≤ 1.5×ULN;ALT and AST ≤ 2×ULN;serum creatinine≤1.5mg/dL;
8. Female subjects of childbearing age must exclude pregnancy and are willing to use a medically approved high-efficiency contraceptive (eg, IUD, contraceptive or condom) during the study period and within 3 months of the last study drug administration;
9. The subject should be aware of the purpose of the study and the operations required by the study and volunteer to participate in the study before sign the informed consent form;
Exclusion Criteria:
1. Concurrent administration of any other antitumor therapy；
2. Allergic to the ingredients of research drugs；
3. Has received prior therapy with an anti-PD-1, anti-PD-L1, or anti-PD-L2 agent；
4. Any active autoimmune diseases or a history of autoimmune diseases (including but not limited to: autoimmune hepatitis, interstitial pneumonia, uveitis, enteritis, hepatitis, pituitary inflammation, vasculitis, nephritis, hyperthyroidism, decreased thyroid function；
5. Systemic steroid therapy or any other form of immunosuppressive therapy within 7 days prior to the first dose of trial treatment, except for a maximum dose of 4 mg/day dexamethasone or equivalent doses of other corticosteroids or control of brain edema, which has been stable or decreased for at least 1 week prior to inclusion;
6. Active infection;
7. Risk of bleeding;
8. HIV positivity;
9. Pregnancy and lactation;
