This is an open,multicentre phase Ib/II study. The purpose of phase Ib is to evaluated the safety, tolerability, pharmacokinetics, immunogenicity, and preliminary antitumor activity of SHR-A1811 in combination with chemotherapy and/or immunotherapy in HER2 expression advanced/metastatic gastric/gastroesophageal junction adenocarcinoma patients.

The Phase II study was designed to evaluate the efficacy and safety of SHR-A1811 in combination with chemotherapy and/or immunotherapy for advanced/metastatic HER2 expression gastric/gastroesophageal conjunctional adenocarcinoma patients.

Inclusion Criteria:

1. Age from 18 to 75, Male and female participants
2. Local advanced unresectable or metastatic disease Pathologically documented adenocarcinoma of gastric or gastroesophageal junction;
3. HER2 expression ;
4. Phase Ib: Failed or intolerant to prior standard treatment, or did not receive any systemic anti-tumor therapy; Phase II: previously untreated patients;
5. ECOG 0-1;
6. The expected survival ≥3 months;
7. Sufficient bone marrow and organ functions;

Exclusion Criteria:

1. There were ascites, pleural effusion and pericardial effusion that needed to be treated;
2. Major surgery was performed within 4 weeks;
3. Have active autoimmune disease or history of autoimmune disease;
4. Patients with interstitial pneumonia;
5. There was severe infection within 4 weeks before starting the study treatment;
6. Patients with active pulmonary tuberculosis infection history within 1 year before enrollment;
7. Serious cardiovascular and cerebrovascular diseases;
8. Gastrointestinal perforation and/or gastrointestinal fistula occurred within 6 months before enrollment;