To evaluate the clinical efficacy and safety of Tislelizumab combined with apatinib mesylate, oxaliplatin plus S1 Vs oxaliplatin plus S1.

Inclusion Criteria:

• Age: 18-70 years of age.
• Histologically confirmed gastric adenocarcinoma was diagnosed in patients with locally advanced gastric cancer with tumor volume >5cm Borrmann III, Borrmann IV and BulkyN according to AJCC Version 8.
• Measurable lesions at least should be detected by CT/MRI examination in accordance with the RECIST1.1.
• ECOG（Eastern Cooperative Oncology Group）PS（Performance Status）:0-1 scores.
• No previous surgical treatment, anti-tumor chemoradiotherapy/immunotherapy was performed.
• Preoperative endoscopic examination confirmed no positive peritoneal implantation metastasis and exfoliated cells.
• The expected survival time is more than 6 months.
• For women of reproductive age, a urine or serum pregnancy test with negative results should be performed within 3 days prior to receiving the first study drug administration (day 1 of cycle 1).If a urine pregnancy test result cannot be confirmed as negative, a blood pregnancy test is requested.Women of childbearing age were defined as at least 1 year after menopause or having undergone surgical sterilization or hysterectomy.

Exclusion Criteria:

• Diagnosis of malignant diseases other than gastric cancer within 5 years prior to first administration (excluding radical basal cell carcinoma of the skin, squamous carcinoma of the skin, and/or radical resectable carcinoma in situ).
• Significant clinical bleeding symptoms or clear bleeding tendency, such as gastrointestinal bleeding, hemorrhagic gastric ulcer or vasculitis, etc. occurred within 3 months before enrollment. If fecal occult blood was positive at baseline, reexamination could be performed，if it was still positive after reexamination, gastroscopy was required.
• Prior treatment: anti-PD-1, anti-PD-L1, or anti-PD-L2 drugs or drugs that target another stimulating or co-inhibiting T-cell receptor (e.g., CTLA-4, OX-40, CD137).
• A history of immunodeficiency, including HIV testing positive.
• Is currently participating in an interventional clinical study or has been treated with another study drug or study device in the 4 weeks prior to initial dosing.
• Patients who had a history of cardiovascular and cerebrovascular diseases and were still taking thrombolytic drugs or anticoagulants orally.
• HER2 positive is known.
• Patients with previous gastrointestinal perforation, abdominal abscess or recent intestinal obstruction (within 3 months) or imaging or clinical symptoms suggesting intestinal obstruction.