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.