Key Inclusion Criteria:

1. Age≥18 years old, male or female, signed Informed Consent Form (ICF).
2. Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1
3. Histologically or cytologically confirmed ES-SCLC
4. No prior systemic treatment for ES-SCLC
5. Adequate hematologic and end organ function

Key Exclusion Criteria:

1. Active leptomeningeal disease or uncontrolled, untreated brain metastasis;
2. Prior therapy with an antibody or drug against immune checkpoint pathways, including but not limited to, anti program death receptor-1 (anti-PD-1), anti-PD-L1, or anti cytotoxic T lymphocyte associated antigen 4 (anti CTLA-4) antibody;
3. Was administered a live vaccine ≤ 4 weeks before randomization;
4. Active autoimmune diseases or history of autoimmune diseases that may relapse
5. Any condition that required systemic treatment with either corticosteroids or other immunosuppressive medication ≤ 14 days before randomization;
6. With a history of interstitial lung disease, non-infectious pneumonitis, or uncontrolled systemic diseases;
7. Severe chronic or active infections requiring systemic antibacterial, antifungal or antiviral therapy within 2 weeks prior to randomization, including but not limited to tuberculosis infection;
8. Participant with untreated hepatitis B virus (HBV)/hepatitis C virus (HCV), or a known history of HIV infection;
9. Participants with toxicities (as a result of prior anticancer therapy) which have not recovered to baseline or stabilized at the time of randomization;
10. Clinically significant pericardial effusion, or Clinically uncontrolled pleural effusion

NOTE: Other protocol defined Inclusion/Exclusion criteria may apply.