• Histologically or cytologically confirmed ES-SCLC, defined by the American Joint Committee on Cancer (AJCC) 8th edition or the Veterans Administration Lung Study Group (VALG) staging system.

• No prior treatment for ES-SCLC. (Patients who have received prior chemoradiotherapy for limited-stage SCLC must have been treated with curative intent and experienced a treatment-free interval of ≥ 6 months between the completion of chemotherapy, radiotherapy, or chemoradiotherapy and diagnosis of ES-SCLC).

• ECOG performance status ≤ 1.

• Life expectancy ≥ 3 months.

• Adequate organ function as indicated by the following laboratory values (obtained ≤ 7 days before first dose):

• Absolute neutrophil count (ANC) ≥ 1.5 × 109/L, platelets ≥ 100 × 109/L，hemoglobin ≥ 90 g/L.

• International normalized ratio (INR) or prothrombin time (PT) ≤ 1.5 x upper limit of normal (ULN).

• Activated partial thromboplastin time (aPTT) ≤ 1.5 x ULN.

• Serum total bilirubin ≤ 1.5 x ULN.

• Aspartate and alanine aminotransferase (AST and ALT) ≤ 2.5 x ULN, or AST and ALT ≤ 5 x ULN for patients with liver metastases.

⁃ Serum albumin (ALB) ≥ 25g/L.

⁃ Serum creatinine ≤ 1.5 x ULN or estimated glomerular filtration rate (GFR) ≥ 60 mL/min/1.73 m2 by Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation.

⁃ Able to provide written informed consent by the patient or by the patient's legally acceptable representative and can understand and agree to comply with the requirements of the study.

⁃ 18 to 75 years old on the day of signing the informed consent form (ICF).

⁃ Fertile patients must be willing to use highly effective contraception during the study period and for 120 days after the last dose of tislelizumab.