Inclusion Criteria:
1. Ambulatory males or females with ages ≥ 18.
2. Karnofsky Performance Scale（KPS） ≥ 70.
3. Pathologically confirmed gastric adenocarcinoma (including Lauren classification), with a confirmed HER2-negative status (IHC 0, 1+, or 2+ with FISH negative).
4. A clinical stage of cIII/IVa (AJCC 8th edition gastric cancer TNM staging) diagnosed by enhanced CT/MRI, combined with endoscopic ultrasound (EUS) and diagnostic laparoscopy if necessary.
5. For gastroesophageal junction (GEJ) cancer, only patients with Siewert type III or Siewert type II disease not requiring combined thoracotomy are eligible for enrollment.
6. The tumor can undergo radical resection.
7. The surgeon and the center is experienced and qualified for gastrectomy with D2 lymphadenectomy (the number of lymph nodes examined must at least be 15, so as to ensure the quality of the surgery).
8. Performance status and adequate organ function to tolerate major abdominal surgery.
9. Results of blood routine tests and biochemistry at baseline meet the following standards: hemoglobin (Hb) ≥ 90g/L; absolute neutrophil count (ANC) ≥ 2×10⁹/L; platelet count ≥ 100×10⁹/L; ALT/AST ≤ 2.5×ULN; ALP ≤ 2.5×ULN; serum total bilirubin (STB) \< 1×ULN; serum creatinine \< 1×ULN; and serum albumin ≥ 30g/L.
10. Left ventricular ejection fraction (LVEF) ≥ 50% confirmed by echocardiography.
11. Not having any serious concomitant disease that makes the survival less than five years.
12. Be willing to and able to abide by the protocol throughout the study.
13. Having given written informed consent prior to screening and understood that he or she will be free to withdraw from the study at any time without suffering any loss.
14. Agree to provide blood and histological samples.
Exclusion Criteria：
1. Pregnancy or lactation women.
2. Women of childbearing potential who are positive for the pregnancy test at the baseline or fail to take the test; women after the menopause must have stopped menstruating for at least 12 months that they will be believed to be unable to get pregnant;
3. Sexually active (potentially fertile) males and females who are unwilling to take any method of contraception during the study;
4. Patients who were previously allergic to monoclonal antibody;
5. Patients with active autoimmune diseases;
6. Patients with any signs of distant metastatic lesions;
7. Patients who previously received cytotoxic chemotherapy, radiotherapy or immunotherapy for the treatment of the present gastric adenocarcinoma, excluding corticosteroids;
8. A medical history of other malignant disease in the last five years, excluding cured skin cancer and carcinoma in situ (CIS);
9. Having uncontrolled epilepsy, central nervous system disease or mental disorder, whether which is clinically serious enough to affect signature of informed consent form or the patient's compliance with oral administration of the study drug will be judged;
10. Having clinically serious (active) heart disease, such as symptomatic coronary heart disease (CHD), NYHA (New York Heart Association) II or more serious congestive heart failure, or serious arrhythmia that requires drug interventions, or a medical history of myocardial infraction (MI) during the last 12 months.
11. Patients who have upper gastrointestinal obstruction, or physiological dysfunction, or malabsorption syndrome, which may affect absorption of S-1.
12. Patients with active infections (such as hepatitis A/B/C and HIV).
13. Patients with severe comorbidity, for instance, uncontrolled diabetes.
14. Patients who concurrently receive potent CYP3A inhibitors.
15. Known to have peripheral neuropathy ≥ NCI CTC AE I; but patients who only present loss of deep tendon reflexes (DTRs) may not be excluded;
16. Patients with history of organ transplantation who require immunosuppressive therapy;
17. Patients who suffer from serious uncontrolled repeated infection, or other serious and uncontrolled disease;
18. Moderate or severe renal dysfunction \[creatinine clearance ≤ 50ml/min (calculated according to the Cockroft-Gault formula, please refer to Appendix 4）， or serum creatinine \> the upper limit of normal (ULN);
19. Patients with dihydropyrimidine dehydrogenase (DPD) deficiency;
20. Having received any other study drug or agent / treatment (i.e., participated in other trial) within four weeks prior to randomization).
21. Other patients considered unsuitable for enrollment by the investigator.
