The purpose of this study is to find out whether treatment with Serplulimab combined with Trastuzumab and Chemotherapy will improve the survival of gastric cancer patients with stage II-III after surgery.

Inclusion Criteria:

1. Lower age limit of research subjects 20 years old and upper age limit of 80 years old.
2. Be proven to be primary adenocarcinoma of gastric cancer and staged II-III by pathological evidences
3. R0 gastrectomy with D2 lymphadenectomy
4. Her2+ diagnosed by Immunohistochemistry or FISH
5. ECOG (ECOG score standard) performance status of 0 or 1 and expected to survive more than 6 months
6. No contraindications, including normal peripheral blood routine, liver and kidney function and electrocardiogram （WBC≥3.5 x 109 /L, NEU≥1.2 x 109 /L,PLT≥90 x 109 /L and HGB≥80g/L).

Exclusion Criteria:

1. Patients with stage I and IV.
2. Unavailable for R0 resection and D2 lymph node dissection.
3. Multiple primary tumors
4. Suffering from other serious diseases, including cardiovascular, respiratory, kidney, or liver disease, complicated by poorly controlled hypertension, diabetes, mental disorders or diseases.
5. History of chemotherapy, radiotherapy, immunotherapy or target therapy.