Inclusion Criteria:  Patients must have the ability to understand and voluntarily sign informed consent; Age: over 18 years old; Expected survival period ≥ 3 months; Histologically or cytologically confirmed ES-SCLC (according to the Veterans Lung Administration Lung Study Group, VALG staging); No prior systemic treatment for ES-SCLC; Eastern Cooperative Oncology Group performance status (ECOG PS) 0 or 1; At least 1 measurable lesion by computed tomography (CT) or magnetic resonance imaging (MRI) per Response Eval -uation Criteria in Solid Tumors version 1.1 (RECIST v1.1) criteria; Adequate hematologic and end organ function.  Exclusion Criteria:  Have been previously exposed to any antibody or drug of immune-mediated therapy, including but not limited to LAG-3, anti-cytotoxic T lymphocyte antigen-4 (CTLA-4), anti-PD-1, anti-PD-L1 antibodies. Have received systemic treatment with Chinese herbal medicine or immunomodulatory drugs with anti-tumor indications (including thymosin, interferon, interleukin, except for local use to control pleural effusion) within 2 weeks prior to the first administration of study drug. Have active or uncontrolled central nervous system (CNS) metastases and/or spinal cord compression and/or carcinomatous meningitis, or history of leptomeningeal carcinoma. Subjects with a history of radiotherapy or surgery for brain metastases and asymptomatic CNS metastases at the time of screeing are eligible if they meet all of the following criterias: have measurable lesions outside the CNS; do not have midbrain, pons, meninges, medulla oblongata or spinal cord metastases; do not have evidence of new or enlarged brain metastases after treatment for brain metastases, and corticosteroids and anticonvulsants treatments have been discontinued for at least 14 days prior to the study treatment. Subjects with asymptomatic brain metastases can be included if the brain metastases have been treated with radiotherapy and above mentioned criterias are all met. Are expected to require any other antineoplastic therapy while in study (PCI is allowed). Have received administration of live attenuated vaccines within 4 weeks prior to the first administration of study drug or anticipation that such a live attenuated vaccine will be required during the study.