This is a randomized, double-blind, multicenter, phase III clinical study to compare the clinical efficacy and safety of HLX10+ Chemotherapy vs placebo+Chemotherapy in Previously Untreated Patients with Extensive Stage Small Cell Lung Cancer.

Eligible subjects in this study will be randomized to Arm A or Arm B at 2:1 ratio as follows:

Arm A (HLX10 arm): HLX10 + chemotherapy (Carboplatin-Etoposide) ; Arm B (placebo arm): Placebo + chemotherapy (Carboplatin-Etoposide); The three stratification factors for randomization include: PD-L1 expression level (negative, positive, not available), Brain metastasis (yes versus no), Age (≥ 65 years versus < 65 years)

After screening, subjects meeting the inclusion criteria and none of the exclusion criteria will be enrolled. Included subjects will be treated with HLX10 or placebo in combination with chemotherapy once every 3 weeks, until disease progression, death, intolerable toxicity, withdrawal of informed consent, or occurrence of other reasons specified in the protocol (whichever occurs first).

Inclusion Criteria:

• Histologically or cytologically diagnosed with ES-SCLC (according to the Veterans Administration Lung Study Group staging system).
• No prior systemic therapy for ES-SCLC
• Major organs are functioning well
• Participant must keep contraception

Exclusion Criteria:

• Histologically or cytologically confirmed mixed SCLC.
• Known history of severe allergy to any monoclonal antibody.
• Known hypersensitivity to carboplatin or etoposide.
• Pregnant or breastfeeding females.
• Patients with a known history of psychotropic drug abuse or drug addiction.
• Patients who have other factors that could lead to the early termination of this study based on the investigator's judgment.

From ICF until 90 days after the last study treatment (up to approximately 56 months).

All AEs and treatment emergent AEs are recorded from the time of signing the ICF until 90 days after the last study treatment. If a subject starts a new antineoplastic therapy during the AE collection period, only information on AEs related to study treatment are collected after the new antineoplastic therapy.