Inclusion Criteria:  Meet the diagnostic criteria of EBV-HLH or CAEBV. Newly diagnosed patients. Eastern Cooperative Oncology Group score 0-3. Total bilirubin <= 10 times of upper limit of normal (except for those caused by EBV-HLH or CAEBV), and serum creatinine <= 1.5 times of upper limit of normal. Serum HIV antigen or antibody is negative. Hepatitis C virus (HCV) antibody is negative, or HCV antibody is positive, but HCV-RNA is negative. Both HbsAg and HbcAb for Hepatitis B virus (HBV) are negative. If not, the quantification of HBV-DNA needs to be <1*10e3 IU/ml. The patient or his/her guardian must be able to understand and be willing to participate in this study, and sign the informed consent.  Exclusion Criteria:  Non EBV-HLH or CAEBV patients. Refractory or relapsed EBV-HLH or CAEBV. Heart function above grade II (NYHA). Patients suffered from other uncontrollable active infections. Pregnant or lactating women. Patients with mental disorders and cannot cooperate with the requirements of research, treatment and monitoring. Active visceral bleeding. Allergic to PD-1 monoclonal antibody. Patients with known autoimmune diseases. Participate in other clinical research at the same time. The investigator judges that the patient has other reasons not suitable for this study, or participating in the study will bring great risks to the patient.