Inclusion Criteria:  Males and females aged 18-75 years; Histologically or cytologically confirmed esophageal small cell carcinoma with unresectable locally advanced / recurrent or distant metastasis Patients who have not received systemic anti-tumor therapy Patients with recurrence or metastasis more than 6 months after the end of adjuvant or neoadjuvant chemotherapy accompanied by radical surgery or radical chemoradiotherapy; With at least 1 measurable lesion according to RECIST 1.1 criteria; ECOG score 0-1; Expected survival ≥3 months; Good organ function (without blood transfusion, use of hematopoietic stimulating factors, or transfusion of albumin or blood products within 7 days prior to examination): 1) Platelet (PLT) count ≥75,000 /mm3; 2) Neutrophil count (ANC) ≥1,500 /mm3; 3) Hemoglobin (Hb) level ≥9.0 g/dl; 4) Total bilirubin (TBIL) level ≤1.5×ULN; 5) Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) level ≤2.5×ULN (≤5×ULN in case of liver metastasis); 6) Alkaline phosphatase level ≤2.5×ULN (≤5×ULN in case of liver metastasis); 7) Serum creatinine (Cr) level ≤1.5×ULN and creatinine clearance >50 ml/min; Females of child bearing age must have anegative pregnancy test, and have to take contraception measures and for 3 months after the last dose Able to understand and willing to sign written informed consent form. Patients who agree to provide previously stored tumor tissue samples or perform biopsy to collect tumor tissue for gene testing.  Exclusion Criteria:  Known allergy to study drug or excipients, or allergy to similar drugs; Received anti-tumor cytotoxic drug therapy, biological drug therapy (such as monoclonal antibody), immunotherapy (such as interleukin-2 or interferon) or other research drug therapy within 4 weeks before enrollment. Received tyrosine kinase inhibitor treatment within 2 weeks before enrollment. Patients received radiotherapy within 4 weeks or radiopharmaceuticals within 8 weeks, except for local palliative radiotherapy for bone metastases. Major surgery was performed or not completely recovered from the previous surgery within 4 weeks before enrollment (the definition of major surgery refers to the level 3 and level 4 surgery specified in the administrative measures for clinical application of medical technology implemented on May 1, 2009). The toxicity of previous anti-tumor therapy has not recovered to CTCAE [version 4.03] 0-1, except for the following cases: a) lipsotrichia；b) Pigmentation;c) Peripheral neurotoxicity has recovered to < CTCAE 2;d) The long-term toxicity caused by radiotherapy could not be recovered according to the judgment of the researchers; Subjects with clinically symptomatic CNS metastases and/or cancerous meningitis. The subjects who have received brain or meningeal metastasis treatment in the past, if the clinical stability has been maintained for at least 2 months, and the systemic hormone treatment has been stopped for more than 4 weeks can be included. Have or are currently suffering from other malignancies (except for non melanoma basal cell carcinoma of the skin, breast / cervical carcinoma in situ, and other malignancies that have been effectively controlled without treatment in the past five years). Subjects have any active autoimmune disease or history of autoimmune disease (including but not limited to: interstitial pneumonia, uveitis, enteritis, hepatitis, hypophysitis, nephritis, hyperthyroidism, hypothyroidism; subjects with vitiligo or asthma in childhood who have completely remission and do not need any intervention in adulthood can be included; subjects with asthma requiring bronchodilator for medical intervention can not be included). Previous use of anti-PD-1 antibody, anti-PD-L1 antibody, anti-PD-L2 antibody or anti-CTLA-4 antibody (or any other antibody acting on T cell costimulation or checkpoint pathway). Subjects with active pulmonary tuberculosis (TB) are receiving antituberculosis treatment or received antituberculosis treatment within one year before screening. Patients with complications requiring long-term use of immunosuppressive drugs or systemic or local use of corticosteroids with immunosuppressive effect (dose > 10mg / day of prednisone or other therapeutic hormones). Received any anti infection vaccine (such as influenza vaccine, varicella vaccine, etc.) within 4 weeks before enrollment. Pregnant or lactating women. HIV positive. HBsAg positive and HBV DNA copy number positive (quantitative detection ≥ 1000 CPS / ml). HCV antibody positive. Researchers believe that it can affect the compliance of the protocol, or affect the subject to sign the informed consent(ICF), or any other disease or condition of clinical significance that is not suitable to participate in this clinical trial. There are clinical symptoms or diseases that can not be well controlled, such as: (1) heart failure of NYHA grade 2 or above (2) unstable angina pectoris (3) myocardial infarction within one year (4) clinically significant supraventricular or ventricular arrhythmia requiring treatment or intervention.