Inclusion Criteria:
1. Histologically confirmed gastric adenocarcinoma with her 2 (-), and without the history of resection of primary or metastatic lesion;
2. Peritoneal metastases from gastric cancer requiring definitive diagnosis by laparoscopy, and without gastric outflow tract obstruction and intestinal obstruction; Written (signed) informed consent;
3. Claudin 18.2 positive (≥ 25%, and the proportion of positive cells greater than 50% of the cases accounted for not less than 70%);
4. Age ≥ 18 years at registration;
5. Eastern Cooperative Oncology Group (ECOG) score ≤ 1;
6. Expected life expectancy \> 3 months;
7. Adequate bone marrow, liver, and renal functions.
Exclusion Criteria:
1. Confirmed of evidence of distant metastasis other than peritoneal metastasis (e.g.liver metastasis, lung metastasis, para-aortic lymph node metastasis, etc.);
2. During pregnancy, within 28 days of post parturition, or during lactation;
3. Previously received immunotherapy such as PD-1/PD-L1 and CTLA-4 or targeted therapy such as Claudin 18.2.
4. Synchronous or metachronous (within 5 years) malignancies.
5. Severe mental disease, uncontrolled epilepsy, or central nervous system disease;
6. Clinically severe (i.e. active) heart disease, such as symptomatic coronary heart disease, New York Heart Association (NYHA) class II or more severe congestive heart failure or arrhythmia requiring drug intervention, or a history of myocardial infarction in the last 12 months;
7. Upper gastrointestinal obstruction or abnormal physiological function or malabsorption syndrome may affect S-1 absorbers;
8. Known peripheral neuropathy (\> NCI-CTC AE 1). However, patients with only disappearance of deep tendon reflex (DTR) need not be excluded;
9. Patients on steroid or immunosuppressant treatment after organ transplant;
10. Patients with severe uncontrolled recurrent infections or other severe uncontrolled concomitant disease;
11. Moderate or severe renal damage \[creatinine clearance ≤ 50 ml/min\], or serum creatinine \> upper limit of normal (ULN), 115 μmol/L;
12. Known dihydropyrimidine dehydrogenase (DPD) deficiency;
13. Anaphylaxis to paclitaxel or any research drug ingredient.
14. Active autoimmune disease or history of refractory autoimmune disease;
15. Subjects with hypothyroidism requiring only hormone replacement therapy and skin diseases without systemic treatment (such as vitiligo, psoriasis or alopecia) can be selected;
16. HIV antibody positive, active hepatitis B or C (hepatitis B: HBsAg positive and HBV DNA ≥10 copies/ml; hepatitis C: HCV antibody and HCV-RNA positive, requiring antiviral treatment at the same time);
17. Steroid or other systemic immunosuppressive therapy was used 14 days before admission, excluding local or physiological doses of systemic glucocorticoids (eg. no more than 10mg/day of prednisone or other glucocorticoids of equivalent dose) by nasal spray, inhalation or other routes, or hormones used to prevent allergy of contrast agents;
18. Uncontrolled arrhythmia and myocardial infarction within 12 months before admission or active tuberculosis.
