A Prospective, Single Center Clinical Study of Toripalimab Combined With FLOT Regimen for Perioperative Treatment of PD-L1 Positive Locally Advanced Resectable Gastric Cancer or Gastroesophageal Junction Adenocarcinoma (GEJ)

Inclusion Criteria:

1. Informed consent of the patient;
2. 18 years old \<age \<75 years old;
3. The researcher judged that he could comply with the study protocol;
4. Histologically confirmed gastric or gastroesophageal junction adenocarcinoma (Siewert II-III);
5. Clinical stage at admission: ≥T3 with ≥1 lymph node metastasis and no distant metastasis (AJCC 8th)

   * Esophagogastroduodenoscopy must be performed
   * Diagnostic laparoscopy must be performed
6. Immunohistochemistry confirmed that the patient was PD-L1 positive (CPS≥1);
7. Preoperative ECOG \[Using the ECOG scoring standard Zubrod-ECOG-WHO (ZPS, 5-point method) developed by the Eastern Cooperative Oncology Group (ECOG)\] Physical State Score 0/1;
8. Preoperative ASA score I-III;
9. Expected survival ≥12 weeks;
10. The baseline blood routine and biochemical parameters of selected patients should meet the following criteria:

    * Hemoglobin ≥90g/ L, which can be met by blood transfusion;
    * Absolute neutrophil count ≥1.5×10\^9/ L
    * Platelet count ≥100×10\^9/ L
    * Aspartic acid or alanine aminotransferase ≤ 2.5 times the upper limit
    * Alkaline phosphatase ≤ 2.5 times normal upper limit (ULN),
    * Thyroid stimulating hormone (TSH) ≤1 times ULN (if abnormal, T3 and T4 levels should be investigated at the same time; if T3 and T4 levels are normal, they can be included in the group);

Exclusion Criteria:

1. Inestigators identified stage IV (metastatic) or unresectable gastric or gastroesophageal junction adenocarcinoma
2. Previous systemic treatment for gastric cancer
3. Have a history of allergy to any component of terriprizumab, oxaliplatin, capecitabine;
4. Received any of the following medical treatment:

   A. Have received anti-PD-1 or anti-PD-L1 antibody therapy in the past; B. Have received any investigational drug treatment within 4 weeks before using the drug for the first time; C. Enroll in another clinical study at the same time, unless it is an observational (non-interventional) clinical study or an interventional clinical study follow-up; D. Receive the last dose of anti-cancer treatment (including radiotherapy) within 4 weeks before the first use of the study drug;; E. Receive systematic treatment with corticosteroids (\> 10mg prednisone equivalent daily dose) or other immunosuppressants within 2 weeks prior to their first use of the study drug; F. Those who have been vaccinated with anti-tumor vaccines or the study drugs have been vaccinated with live vaccines within 4 weeks before the first administration; G. Those who have undergone major surgery or trauma within 4 weeks before using the study drug for the first time;
5. History of other malignant diseases within 5 years;
6. History of active autoimmune disease or autoimmune disease
7. The subject has cardiovascular clinical symptoms or disease that is not well controlled;
8. Severe infection 4 weeks prior to first use of study drug (CTCAE \> Level 2)
9. A history of interstitial lung disease (except for radiation pneumonia not treated with Chinese hormone) or non-infectious pneumonia;
10. Patients with active pulmonary tuberculosis infection found by history or CT examination, or patients with active pulmonary tuberculosis infection history within 1 year before enrollment, or patients with active pulmonary tuberculosis interference history more than 1 year ago but without formal treatment;
11. Pregnant or lactating women;
12. Suffering from severe mental illness.