Inclusion Criteria:

          -  1. Patients must sign a specific informed consent form prior to clinical trial; 2.
             Extensive stage small cell lung cancer confirmed by histology or cytology; 3. Patients
             did not receive any system treatment before or only received EP/EC chemotherapy (time
             from the last medication of chemotherapy to the beginning of maintenance treatment
             must be ≤21 days); 4. Eastern Cooperative Oncology Group (ECOG) performance status
             0-1; 5. An estimated survival duration of >5 months from the beginning of
             chemotherapy; 6. Age no less than 18; 7. A measurable lesion on image; 8. Patients
             with asymptomatic brain metastases or symptomatic brain metastases which were stable
             after treatment; 9. Before the first dose of drugs for study, patients should have
             appropriate organ function and the laboratory results must meet conditions as
             following: Blood routine examination: neutrophil absolute value (ANC) ≥1.5×109/L,
             platelet (PLT) ≥100×109/L, hemoglobin content (HGB) ≥9g/dl; Adequate hepatic function:
             bilirubin ≤1.5×ULN mg/dl, creatinine clearance ≥ 50 ml/min; Adequate hepatic function:
             Aspartate aminotransferase (AST) /alanine aminotransferase (ALT)> 2.5 × upper limit of
             normal (ULN) or > 5 × ULN (patients with liver metastasis), alkaline phosphatase (ALP)
             ≤2.5×ULN or ≤5×ULN (patients with bone metastasis), Total bilirubin (TB) ≤1.5×ULN,
             albumin (ALB)>30g/dl; Coagulation function: international normalized ratio (INR)
             ≤1.5×ULN, activated partial thromboplastin time (APTT) ≤1.5×ULN; Urine routine:
             24-hour urine protein<1g (if urine protein ≥2+, additional 24-hour urine protein is
             required); Others: serum lipase or amylase ≤1.5×ULN or >1.5×ULN (subjects clinically
             or radiologically diagnosed with pancreatitis).

        Exclusion Criteria:

          -  1. Patients received EP/EC regiment received the last medication ≥21 days before
             maintenance treatment, or received other systematic anti-tumor treatment for ES-SCLC;
             2. Patients with epidermal growth factor receptor (EGFR) or anaplastic lymphoma kinase
             (ALK) mutation; 3. Patients received any other experimental drugs or participated in
             another interventional clinical study within 4 weeks prior to signing the informed
             consent; 4. Patients received other systemic or local antitumor therapy (including but
             not limited to the use of other drugs for SCLC maintenance therapy or radiotherapy,
             but CR/PR subjects are allowed to use prophylactic cranial irradiation (PCI) after
             induction period treatment); 5. Patients with active and untreated brain metastases or
             carcinoma meningitis in CT or MRI examination during screening stage; 6. Patients with
             other malignant tumors within 5 years, except for curable malignant tumors (carcinoma
             in situ or stage I tumor), such as cervical carcinoma in situ, basal cell or squamous
             cell skin cancer and so on); 7. Patients received corticosteroids (>10mg/ day methyl
             prednisolone or equivalent dose) or other immunosuppressants (inhalation or local use
             of steroids and adrenal replacement treatment were permitted in the absence of an
             active autoimmune disease) less than 14 days prior to maintenance medications; 8.
             Patients with chronic or acute active hepatitis B (HBsAg positive and hepatitis B
             virus (HBV) DNA copy number >ULN), or HCV positive (HCV Ab positive and HCV RNA
             positive); Hepatitis B patients with previous HBV infection or who have been cured
             (HBsAg negative, HBcAb positive and HBV DNA copy number < ULN) were allowed to be
             enrolled; 9. Patients with interstitial lung disease, drug-induced pneumonia,
             radiation pneumonitis requiring steroid treatment, or active pneumonia with clinical
             symptoms; or other lung diseases causing moderate or severe lung dysfunction; Active
             pulmonary tuberculosis or the need for anti-tuberculosis treatment; 10. Patients who
             were allergy to one of research drugs, or allergy to any one of the immunocheckpoint
             inhibitors or other platinum; 11. Female patients during pregnant and lactation
             period, or patients were plan to pregnant; 12. Patients with factors that may cause
             the study to be forced to terminate halfway according to investigators' judgement,
             such as poor compliance, other serious diseases requiring combined treatment and so
             on.