Tislelizumab Combined With Neoadjuvant Chemotherapy Used in the Perioperative Treatment.

Inclusion Criteria: * The patient voluntarily joined this study and signed an informed consent form; * Age ≥ 18 years old, ≤ 75 years old * Pathological diagnosis of gastric adenocarcinoma or adenocarcinoma of the gastroesophageal junction * Patients must be able to provide fresh slices of tumor tissue (FFPE tissue blocks or approximately 15 slides), unstained FFPE slides, and if clinically feasible, fresh biopsy samples will be preferred. If archived samples cannot be obtained, fresh tumor biopsy specimens must be collected during the baseline period, with the same requirements for glass slides as archived tumor tissue * Clinical staging determined by CT and laparoscopy for curative resection is cT3\~4aN+M0 or cT4bNanyM0 for gastric and gastroesophageal junction adenocarcinoma patients (according to AJCC 8th edition staging) * Have not received anti-tumor treatment (such as surgery, radiotherapy, chemotherapy, targeted therapy, immunotherapy, etc.) * Plan to undergo surgical treatment after the completion of neoadjuvant therapy * Able to swallow pills normally * ECOG score 0-1 points * Expected survival time ≥ 12 months * Normal function of major organs, i.e. meeting the following criteria:The standard for blood routine examination must comply with: (No blood transfusion or blood products within 14 days, no correction using G-CSF or other hematopoietic stimulating factors)，Neutrophil absolute count ≥ 1.5 × 109/L; Platelets ≥ 80 × 109/L; Hemoglobin ≥ 80g/L * Biochemical exam…