SOX Combined With Sintilimab and Trastuzumab Versus SOX Regimen in the Perioperative Treatment of HER2-positive Locally Advanced Gastric Adenocarcinoma

Inclusion Criteria: * Sign the informed consent form. * Locally advanced adenocarcinoma of the stomach or gastroesophageal junction (Siewert type II/III) confirmed by pathology or cytology. * The definition of a positive HER2 test result is as follows: IHC detects HER2 3+ or IHC detects HER2 2+ and FISH is positive. * Clinically, based on chest, abdomen and pelvic CT, gastroscopy, endoscopic ultrasonography, gastrointestinal contrast, ordinary ultrasound, or laparoscopy if possible, it is judged as T3-4a N+ or T4bN any gastric cancer or gastroesophageal junction cancer (refer to AJCC Article Version 8 in stages). * Patients have not received chemotherapy and/or immunotherapy and/or trastuzumab treatment and/or radiotherapy in the past. * Age 18-75 years old. * The Eastern Cooperative Oncology Group (ECOG) performance status score was 0 or 1, and there was no deterioration within 2 weeks before the first administration of the study drug. * Good organ function: Blood routine: hemoglobin ≥90g/L, white blood cell ≥3.0×109/L, neutrophil ≥1.5×109/L, platelet ≥100×109/L; Renal function: creatinine≤1.5×upper limit of normal (UNL) or creatinine clearance ≥60ml/min; Liver function: total bilirubin (TBIL)≤1.5×upper limit of normal (UNL); ALT≤2.5×UNL, AST≤2.5×UNL. Exclusion Criteria: * The pathology is other types besides adenocarcinoma, such as squamous cell carcinoma, adenosquamous carcinoma, neuroendocrine carcinoma and so on. * Have received chemotherapy and/or radiotherapy in the past. * Have received any anti-PD-1, anti-PD-L1/L2 antibodies, anti-CTLA-4 antibodies and other immunotherapy in the past. * Have received any anti-HER2 therapy in the past. * Intra-abdominal dissemination or distant metastasis (M1). * Clinically significant ascites. * Known to have allergic reactions to oxaliplatin and any ingredients or excipients of Tiggio. * Known to have allergic reactions to any ingredients or excipients of Sintilimab and Trastuzumab. * Inability to swallow, intestinal obstruction, or other factors that affect the administration and absorption of the drug.