The study is a prospective, multi-center, open-label, randomized, and controlled phase II clinical trial.

Inclusion Criteria:

1. Histologically/cytologically confirmed diagnosis of SCLC.
2. Radiologically (including brain CT/MRI, chest and abdomen contrasted CT and bone scintigraphy. PET/CT is recommended) confirmed limited-stage.
3. Patients should be ≥ 18 years old.
4. ECOG performance status of 0-1 (Karnofsky performance status ≥ 80).
5. With adequate cardiac, pulmonary, bone marrow, hepatic and renal function.
6. With weight loss no more than 10% within 6 months before diagnosis.
7. Informed consent must be signed.

Exclusion Criteria:

1. Histology confirmed the mixed NSCLC components;
2. Other primary malignant tumors appeared within 5 years before the first administration of the study drug, except for locally curable malignant tumors after radical treatment (such as basal or squamous cell skin cancer, superficial bladder cancer or prostate, cervical or breast carcinoma in situ, etc.);
3. Any disease or condition contraindicated by radiotherapy or chemotherapy;
4. Malignant pleural effusion and pericardial effusion;
5. Pregnant and lactating women;
6. History of idiopathic pulmonary fibrosis (IPF), including pneumonia and organic pneumonia;
7. Received live vaccination within 28 days before the first administration of the study drug;
8. 28 days prior to the first administration of the study drug, he participated in any other drug clinical trials or is undergoing other clinical trials.
9. Have received any antibody / drug (including PD-1, PDL1, CTLA4, tim3, Lag3, etc.) targeting T-cell co regulatory protein (immunocheckpoint).
10. The investigator believes that the subject's complications or other circumstances may affect the compliance with the protocol or may not be suitable for the study.