Inclusion Criteria:

          1. Male or female patient, age≥18 and≤65;

          2. Eastern Cooperative Oncology Group (ECOG) performance status ≤2;

          3. The life expectancy of greater than 12 weeks;

          4. Participants must have histologically or cytologically confirmed metastatic or
             extended disease of SCLC (ED-SCLC).

          5. According to RECIST1.1, participants must have been confirmed Disease progression
             (within 6 months, confirmed by imaging test)after platinum-based doublet chemotherapy
             OR after (PFS>6 months, ) platinum-based doublet chemotherapy and refused to continue
             chemotherapy, OR after second-line or more lines of systemic chemotherapy limited
             disease SCLC (LD-SCLC) patients with disease progression after Synchronous
             chemoradiotherapy, must receive firstline systematic platinum-based doublet
             chemotherapy and refused to receive chemotherapy again.

          6. Evaluable or measurable lesion is required, defined as at least one lesion (not brain
             metastasis) that can be accurately measured based on RECIST 1.1;

          7. Participant need to provided tumor tissue (from an archival tumor sample obtained
             within 1 year or from a new biopsy sample) for PD-L1 immunohistochemical (IHC) assay,
             and PD-L1expression in more than 1% cells is required;

          8. Participant is able to the ability to swallow oral medications

          9. Participants have to meet the following criteria to ensure function of vital organs:

             Absolute neutrophil count (ANC) ≥1.5×109/L or White blood cell count
             >3.5×109/L;Platelets >80×109/L; Hemoglobin (HGB)≥90 g/L;Serum total bilirubin ≤ 1.5 x
             upper limit of normal (ULN); AST(SGOT)/ALT(SGPT) ≤2.5 ×ULN; ALB≥2.8g/dL;Serum
             creatinine ≤ 1.5 x institutional ULN OR creatinine clearance ≥40 mL/min using the
             Cockcroft-Gault equation

         10. Participants must agree to use adequate contraception (hormonal or barrier method of
             birth control; abstinence) through the treatment, and for at least 180 days after the
             last dose of study treatment; Participants must have the ability to understand and be
             willing to sign a written informed consent document.

        Exclusion Criteria:

          1. Participants who were diagnosed as mixed pathological type of small cell lung cancer

          2. Participants who had long-term use of metformin (>2 weeks) 6 months prior to study
             entry, or diagnosed with type-2 diabetes,

          3. Participants received treatment with anti-PD1, -PDL1, -CTLA4, -CD137 inhibitors
             before, or any therapy specifically targeting T-cell co-stimulation or checkpoint
             pathways.

          4. Participants received cellular immunotherapy before

          5. Participants with Uncontrolled intercurrent illness including, but not limited to:

             Ongoing or active infection; Known history of Human Immunodeficiency Virus (HIV)
             infection Acute or chronic active hepatitis B (HBV DNA >1*10^3 copies/ml or >200
             IU/mL) or, acute or chronic active hepatitis C (with a positive Hepatitis C antibody
             test result) Active tuberculosis Congestive heart failure (Class III-IV, according to
             New York Heart Association classification), or and clinically significant Cardiac
             arrhythmia if poorly controlled; Uncontrolled arterial hypertension (systolic blood
             pressure ≥160mmHg or diastolic blood pressure ≥100mmHg) Any arterial thrombosis,
             embolism, ischemia, myocardial infarction, unstable angina, or cerebrovascular
             accident within 6 months prior to enrollment,

          6. Participants with symptomatic Central nervous metastasis or meningeal carcinomatosis
             are excluded; Participants with asymptomatic brain metastases or with brain metastases
             that have been treated and stable in a subsequent scan are allowed to include if there
             is measurable lesion outside the Central nervous system and no history of intracranial
             hemorrhage, and not midbrain, pons, cerebellum, medulla or spinal cord metastasis, and
             do not need glucocorticoid therapy.

          7. Participants receiving glucocorticoid (>30 mg prednisone equivalent a day) or any
             Immunosuppressive drug within 14 days prior to study recruitment; Participants
             receiving inhaled or Topical corticosteroids, adrenal corticosteroid replacement
             therapy (>10 mg prednisone equivalent a day) are allowed if they have no active
             autoimmune disease

          8. Participants with a known additional malignancy (Except for Non-melanoma skin cancer
             and the following in situ carcinoma: in situ bladder carcinoma, in situ gastric
             carcinoma, in situ colonic carcinoma, in situ endometrial carcinoma, in situ cervical
             carcinoma /dysplasia, in situ melanoma carcinoma and in situ breast Carcinoma) unless
             they Maintained Complete Remission for at least 5 years and do not need corresponding
             treatment during the study

          9. Participants who have not recovered (i.e., ≤ Grade 1 according to NCI CTCAE V4or at
             baseline) from adverse effects due to a previously administered agent.

         10. Participants who have uncontrollable effusion, such as pleural and ascites that cannot
             be controlled by drainage or other treatment

         11. Patients who have active autoimmune diseases; excluding patients whose active
             autoimmune disease is caused by Vitiligo or asthma that is completely relieved in
             childhood and Patients with hypothyroidism requiring only hormone replacement therapy

         12. Patients with known Allogeneic organ transplantation (except corneal transplantation)
             or allogeneic hematopoietic stem cell transplantation

         13. Patients who are pregnant or breastfeeding,

         14. Patients who are allergic to monoclonal antibody drugs

         15. Patients who have contraindications to metformin including severe allergic reactions
             and intolerance

         16. Patients who are not eligible for this study, as Assessed by Investigator