Inclusion Criteria:  Histologically confirmed non-keratinizing nasopharyngeal carcinoma, type of WHO II or III. Tumor staged as II-IVa (AJCC 8th,excluding T2N0 disease). Age ≥ 18 years and ≤ 70 years, both genders. Eastern Cooperative Oncology Group (ECOG) performance status (PS) 0 or 1. Patients with plasma EBV DNA> 0 copy/mL or PD/SD according to RECIST1.1 after two cycles of induction chemotherapy. Completed protocol-specified curative chemoradiotherapy, including two cycles of induction chemotherapy, intensity-modulated radiotherapy, and concurrent cisplatin chemotherapy( at least 2 cycles of concurrent cisplatin chemotherapy). Completion of the last radiation dose within 1 to 7 days before randomization No progression after prior cCRT Adequate marrow function: neutrocyte count≥1.5×10e9/L, hemoglobin ≥90g/L and platelet count ≥100×10e9/L. Alanine Aminotransferase (ALT)/Aspartate Aminotransferase (AST) ≤2.5×upper limit of normal (ULN), and bilirubin ≤ 1.5×ULN. Adequate renal function: creatinine clearance rate ≥ 60 ml/min (Cockcroft-Gault formula). Patients must be informed of the investigational nature of this study and give written informed consent. Women of childbearing potential (WOCBP) who are sexually active must be willing to adhere to effective contraception during treatment and for 1 year after the last dose of study drug. Men who are sexually active with WOCBP must be willing to adhere to effective contraception during treatment and for 1 year after the last dose of the study drug.  Exclusion Criteria:  Histologically confirmed keratinizing squamous cell carcinoma (WHO I). Prior malignancy within 5 years, except in situ cancer, adequately treated non-melanoma skin cancer, and papillary thyroid carcinoma. Has received a live vaccine within 30 days before informed consent or will receive a live vaccine in the near future. Is pregnant or breastfeeding. Hepatitis B surface antigen (HBsAg) positive and hepatitis B virus DNA >1×10e3 copies/ml or 200IU/ml Hepatitis C virus (HCV) antibody positive Has active autoimmune disease, except type I diabetes, hypothyroidism treated with replacement therapy, and skin disease that doesn't require systemic treatment (e.g., vitiligo, psoriasis, or alopecia). Has any condition that required systemic corticosteroid (equivalent to prednisone >10mg/d) or other immunosuppressive therapy within 28 days before informed consent. Patients received systemic corticosteroid equivalent to prednisone ≤10mg/d, inhale or topical corticosteroid will be allowed. Has a known history of active TB (bacillus tuberculosis) within 1 year; patients with adequately treated active TB over 1 year ago will be allowed. Has known allergy to large molecule protein products or any compound of sintilimab. Has a known history of interstitial lung disease. Any other condition, including symptomatic heart failure, unstable angina, myocardial infarction, active infection requiring systemic therapy, mental illness or domestic/social factors, deemed by the investigator to be likely to interfere with a patient's ability to sign informed consent, cooperate and participate in the study, or interferes with the interpretation of the results.