Study of Comparing of With and Without Sequential Therapy of S-1

Inclusion Criteria: * (1) Histologically or cytologically confirmed adenocarcinoma of the stomach or adenocarcinoma of the gastroesophageal junction; * (2) Radical surgery (D2 lymph node dissection) for gastric cancer in our center, with postoperative pathological stage of stage III; * (3) No significant tumor recurrence or metastasis as assessed by imaging; * (4) Age 18 years - 75 years; * (5) ECOG score of 0 or 1; * (6) Laboratory tests tolerant to chemotherapy; * (7) Hematological examination: no obvious signs of hematologic diseases, ANC ≥ 1.5 × 10\^9/L, platelet count ≥ 80 × 10\^9/L, Hb ≥ 90 g/L, WBC ≥ 3.0 × 10\^9/L before enrollment, and no bleeding tendency; (8) Biochemical examination: total bilirubin \< 1.5 times the upper limit of normal value, AST, ALT \< 2.5 times the upper limit of normal value, creatinine \< 1.5 times the upper limit of normal value. Exclusion Criteria: * (1) Other pathological types of tumors; * (2) Pregnant or nursing women; * (3) Those with a history of other malignant neoplastic disease in the last 5 years; * (4) Those with a history of uncontrolled epilepsy, central nervous system disease, or psychiatric disorder, where the investigator determines whether the clinical severity prevents signing an informed consent form or affects the patient's compliance with oral medications; * (5) 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 pharmacologic intervention, or a history of myocardial infarction within the last 12 months; * (6) Severe diabetes-related complications, such as diabetic nephropathy, diabetic ketosis, etc; * (7) Those with digestive tract obstruction or physiological abnormalities, or suffering from malabsorption syndrome, which may affect S-1 absorption; * (8) Those who have had gastrointestinal bleeding in the last two weeks, or are at high risk of bleeding as judged by the investigator; * (9) Known to have peripheral nerve disease ≥ NCI-CTC AE grade 1. but with deep tendon reflexes (DTR) only; * (10) Those requiring immunosuppressive therapy for organ transplantation; * (11) who have received other chemotherapy regimens * (12) Those with uncontrolled severe infections, or other serious concomitant diseases; * (13) Allergic to S-1 or any of the study drug components.