Decitabine and Anti-PD-1 in R/R DLBCL

Inclusion Criteria: * 14-80 years old, male or female; * Pathologically confirmed relapsed/refractory diffuse large B cell lymphoma with extranodal (central nervous system) involvement; * Expected survival of more than 3 months; * AST and ALT ≤ 3.0 ULN; TBIL and CCr ≤ 1.0 ULN; * Inactive infection and severe mental illness * ECOG score 0\~2 * According to the New York Heart Association (NYHA) cardiac function grading standards, the heart function grading should be grade I or Grade II; Cardiac ejection fraction \>50% or not lower than the lower limit of the range of laboratory test values at the study center; No pathological abnormality was found in ECG; There was no clinically significant pericardial effusion or pleural effusion * The serum pregnancy test of female subjects must be negative * Signed informed consent Exclusion Criteria: * Subjects with any autoimmune disease requiring long-term use of corticosteroids or immunosuppressive drugs or with a history of other tumors; * Severe uncontrolled medical disease or active infection (including HIV+); * Active gastrointestinal bleeding or 1 month history of gastrointestinal bleeding; * Received allogeneic hematopoietic stem cell transplantation within 6 months or are participating in other clinical studies; * Pregnant or nursing women; * Subjects who must be forcibly detained for the treatment of mental or physical diseases (e.g. infectious diseases); * The researcher thinks it is not suitable for this clinical study (such as poor compliance, drug abuse, etc.) * The situation that the researcher judged was not suitable for inclusion