• Voluntary participation in clinical studies.

• Male or female aged ≥ 18 years at the time of signing the ICF.

• Histologically or cytologically diagnosed with ES-SCLC (according to the Veterans Administration Lung Study Group staging system).

• No prior systemic therapy for ES-SCLC.

• At least one measurable lesion as assessed according to RECIST 1.1 within 4 weeks prior to randomization.

• Major organs are functioning well.

• Every effort should be made to provide tumor tissues for the determination of PD-L1 expression.

• An ECOG PS score of 0 or 1.

• An expected survival ≥ 12 weeks.

• Subjects with prior denosumab use that can and agree to switch to bisphosphonate therapy for bone metastases starting prior to randomization and throughout treatment.

• Participant must keep contraception.