Liver metastases are one of the most common sites of metastasis in advanced gastric cancer. Chemotherapy remains the mainstay of treatment for these patients, but combination chemotherapy has encountered a bottleneck in improving patient survival, with no significant improvement in survival rates at 1, 3 or 5 years. In a previous phase II clinical study we not only observed the survival benefits of Tislelizumab in the treatment of GI tumors such as liver, oesophageal and some gastric cancers, but also confirmed the safety of Tislelizumab in the treatment of advanced GI tumors. This study is a clinical study of PD-1 monoclonal antibody (Tislelizumab) in combination with SOX (Tegafur + Oxaliplatin) for the treatment of liver metastases from gastric cancer. It aims to further explore a new combination therapy for liver metastases from gastric cancer, which is safe and effective for patients with difficult-to-treat disease.

Inclusion Criteria:

• Histologically or cytologically confirmed adenocarcinoma of the stomach or gastroesophageal junction (Siewert type II or III)
• Presence of clearly measurable (meeting RECIST 1.1 criteria) liver metastases on imaging assessment and no more than three metastases with a maximum individual diameter of no more than 5 cm
• No previous systemic treatment for advanced or metastatic gastric cancer
• Age 18 - 75 years old
• Eastern Cooperative Oncology Group physical status score of 0 or 1
• Laboratory Tests Tolerant of Chemotherapy
• Hematological Examination: no obvious signs of haematological disease, ANC ≥ 1.5 × 10^9/L, platelet count ≥ 80 × 10^9/L, Hb ≥ 90 g/L, WBC ≥ 3.0 × 10^9/L prior to enrolment and no bleeding tendency
• Biochemical examination: total bilirubin < 1.5 times the upper limit of normal, AST and ALT < 2.5 times the upper limit of normal, creatinine < 1.5 times the upper limit of normal

Exclusion Criteria:

• Other pathological type of tumor
• Presence of metastases to organs other than the liver
• Pregnant or lactating women
• Those with a history of other malignant neoplastic disease in the last 5 years
• those with a history of uncontrolled epilepsy, central nervous system disease or psychiatric disorder, where the investigator will determine whether the clinical severity prevents the signing of an informed consent or affects the patient's compliance with oral medication;
• Clinically severe (i.e. active) heart disease, such as symptomatic coronary artery disease, New York Heart Association (NYHA) class II or worse congestive heart failure or arrhythmias requiring pharmacological intervention, or a history of myocardial infarction within the last 12 months
• Have severe diabetes-related complications, such as diabetic nephropathy, diabetic ketosis, etc
• Those with digestive tract obstruction or physiological abnormalities, or suffering from malabsorption syndrome, which may affect the absorption of S-1
• Those who have had gastrointestinal bleeding in the last two weeks, or are at high risk of bleeding as judged by the investigator
• Known to have peripheral nerve disease ≥ NCI-CTC AE grade 1. but with deep tendon reflexes (DTR) only
• Those requiring immunosuppressive therapy for organ transplantation
• Presence of any active, known or suspected autoimmune disease
• Those with uncontrolled severe infections, or other severe concomitant diseases
• Those with hypersensitivity to S-1, Oxaliplatin, Tislelizumab or any of the study drug components.