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)