Gastric cancer with retroperitoneal lymph node metastasis was considered as unresectable, to improve these patients' prognosis, we designed systematic conversion therapy including immunotherapy and chemoradiotherapy. The purpose of this study is to estimate safety and efficacy of Sintilimab in combination with chemoradiothrapy followed by D2 surgical resection in patients with advanced gastric cancer with retroperitoneal lymph node metastasis.

Inclusion Criteria:

• Pathology confirmed Gastric/Gastricgastroesophageal junction adenocarcinoma;
• Radiolgical imging including CT,PET-CT or MRI diagnosed as retroperitoneal metastasis.
• Did not receive previous systemic treatment (chemotheray, radiotherapy or both) for advanced disease before.
• ECOG PS 0-2.
• Adequate organ and bone marrow functions and life expectancy ≥12 weeks.

Exclusion Criteria:

• Distant metastases except retroperitoneal metastasis (liver, lung, peitoneal metastasis...);
• HER2-positive status;
• Suspicious active bleeding or gastriointestineal obstruction phenomenon and Has difficulty in swallow tablets and food;
• Has received prior therapy with an anti-programmed death (PD)-1, anti-PD-L1, anti-PD L2 , anti-CD137,anti-CTLA-4 agent or with an agent directed to another stimulatory or co-inhibitory T-cell receptor
• Is currently participating in and receiving study therapy ,except those in the survival follow up period of an investigational agent study or non-interventional study .
• Received systemic treatment with corticosteroids (> 10 mg daily prednisone equivalent) or other immunosuppressive medications within 4 weeks of first dose. Inhaled or topical steroids ,adrenal replacement steroid doses and steroid of prevention allergic reaction of i.v. contrast agent are permitted in the absence of active autoimmune disease.
• Known acute or chronic active hepatitis B infection (positive HBsAg and HBV DNA ≥ 200 IU/mL or ≥ 10^3 copies/mL positive) infection or acute or chronic active hepatitis C (HCV antibody positive and HCV RNA positive) infection.
• Women who are pregnant or nursing.
• Received a live vaccine within 4 weeks of the first dose of study medication or plan to receive live vaccine during study period.
• Active, known or suspected autoimmune disease or has a history of the disease within the last 2 years (subjects with vitiligo, psoriasis, alopecia or Grave's disease, residual hypothyroidism due to autoimmune thyroiditis only requiring hormone replacement, or type I diabetes mellitus only requiring insulin replacement, but not required systemic treatment in the last 2 years, are permitted to enroll) .
• Known primary immunodeficiency.
• Known active tuberculosis.
• Known history of allogeneic organ or allogeneic hemopoietic stem cell transplantation.
• Known>=grade 3 allergy or hypersensitivity to Albumin-paclitaxel oxaliplatin, capecitabine or any monoclonal antibodies.
• Human Immunodeficiency Virus (HIV) infection (HIV antibody positive).
• Poorly controlled arterial hypertension (SBP ≥ 160mmHg or DBP ≥ 100 mmHg) with standard treatment .
• Symptomatic congestive heart failure (New York Heart Association grade II-IV) or symptomatic, poorly controlled arrhythmia.
• Prior arterial thromboembolism event, including myocardial infarction, unstable angina, stroke and transient ischemic attack, within 6 months of enrollment.
• Active or poorly controlled severe infection.
• History of gastrointestinal perforation and /or fistula within 6 months before enrollment.
• Other acute or chronic diseases, mental illness, or abnormal laboratory test results that may lead to the following outcomes: increase the risk of participating in study or study drug administration, or interfere with the interpretation of the study results and considered by investigator as "NOT" eligible to participate in this study.