This study involves patients treated with cisplatin and etoposide (induction therapy), followed by treatment with a drug called S1 (maintenance therapy). The main purpose of this study is to determine if this type of treatment will delay the growth of the tumor and if so, for how long. The primary endpoint is progression free survival and second endpoints are toxicities, overall survival.

Small cell lung cancer (SCLC) accounts for about 20% of lung cancer, has a high degree of malignancy, short doubling time, early and widespread metastasis, is sensitive to chemotherapy and radiotherapy, and has a high initial response rate, but is prone to secondary drug resistance and relapse. The treatment is mainly based on systemic chemotherapy, and if maintenance therapy with S1 after standard EP regimen induction therapy could prolong progression free survival.

Inclusion Criteria:

• 1. Histopathologically confirmed patients with extensive small cell lung cancer; 2. Karnofsky performance status ≥60; 3. At least one lesion that can measured by CT; 4. Expected to survive for at least 3 months; 5. Peripheral blood and liver and kidney function within the following allowable range (tested within 7 days before treatment); 6. White blood cell (WBC) ≥3.0×109/L or Neutrophils (ANC) ≥1.5×109/L; Hemoglobin (HGB) ≥80 g/L；Platelet (PLT) ≥100×109/L; 7. Liver transaminases（AST/ALT）<3.0 times the normal range limit; Total bilirubin（TBIL）<1.5 times the normal range limit; Creatinine（CREAT）<1.5 times the normal range limit; 8. Patients of childbearing age (including female and male patients' partner) must take effective contraception methods; Signed informed consent 9. Must be able to swallow tablets

Exclusion Criteria:

• 1. Limited stage disease 2. Metastastic meningitis, spinal compression, Tumor to main vesicular less than 5mm 3. Other pathological types of tumor except for small cell lung cancer; 4. Patients with a history of severe allergies or allergies; 5. Pregnancy or breastfeeding women; 6. Patients who have previously participated in other clinical trials and have not yet terminated the trial; 7. Patients who have acute infection that difficult to control; 8. Subjects with difficulties in swallowing or known drug malabsorption.