Inclusion Criteria:  Pathologically diagnosed with GC or GEJC, histologically confirmed to be adenocarcinoma, and have no previous anti-tumor treatments for GC or GEJC. Aged 18 or above, male or female. Be suitable for (investigator's assessment) and planning to undergo neoadjuvant therapy + radical surgery with curative intent before entering into the study. locally advanced Gastric Cancer or Gastroesophageal Junction Cancer confirmed by investigator. Be able to provide tumor tissue blocks. Eastern Cooperative Oncology Group (ECOG) performance status score of 0-1. Life expectancy of ≥ 6 months. Have adequate organ and bone marrow functions. Women without childbearing potential refer to post-menopausal women, or women who underwent bilateral oophorectomy with medical records. Male subjects and female subjects of childbearing potential must agree to take a medically approved contraceptive measure (refer to Appendix 4) during the study, within 3 months after the last dose of investigational product (SHR-1701), and within 9 months after the last dose of chemotherapy agents (S-1 and oxaliplatin); have a negative serum pregnancy test result within 3 days prior to the start of study treatment and not be breastfeeding. Subjects must agree and have signed the informed consent form, be willing and able to follow the scheduled visits, study treatment, laboratory tests, and other study procedures.  Exclusion Criteria:  Have known squamous cell carcinoma, undifferentiated carcinoma, or other histological types of gastric cancer, or adenocarcinoma mixed with gastric cancer of other histological types. Have unresectable factors, including unresectable tumors or contraindications to surgery or refusal of surgery. Have more than 20% weight loss within 2 months prior to enrollment or randomization. Have previously received some treatments or medications including anti-tumor treatments. Diagnosed with other malignant tumors within 5 years prior to enrollment. Have any active, known, or suspected autoimmune disease. Have clinically significant bleeding symptoms or clear bleeding tendency within 3 months prior to enrollment or randomization; have gastrointestinal perforation and/or gastrointestinal fistula within 6 months prior to enrollment or randomization; have arterial/venous thrombotic events within 6 months prior to enrollment or randomization. Have major vascular disease within 6 months prior to enrollment or randomization. Have severe, unhealed, or dehisced wounds and active ulcers or untreated fractures. Have intestinal obstruction and/or clinical signs or symptoms of gastrointestinal obstruction within 6 months prior to enrollment or randomization. Have interstitial lung disease, non-infectious pneumonitis, or uncontrolled systemic disease. Have known allergies to the study drug or their excipients; have severe allergic reactions to other monoclonal antibodies. Have HIV infection or known AIDS, active untreated hepatitis or co-infection with hepatitis B and C. Have uncontrolled cardiac symptoms or disease: Have received systemic antibiotics for ≥ 7 days within 4 weeks prior to enrollment or randomization, or have unexplained fever > 38.5 °C during screening or before the first dose. Have known history of allogeneic organ transplantation or allogeneic hematopoietic stem cell transplantation. Have been screened for other clinical studies but failed the screening because PD-L1 expression did not meet the inclusion criteria or met the exclusion criteria; have participated in clinical studies of any other drugs, less than 4 weeks or 5 half-lives of the drug between the last dose of these study treatments and enrollment/randomization for this study (whichever is longer). Have a known history of psychotropic substance abuse or drug abuse. Have other severe physical or psychiatric disorders or laboratory abnormalities, which may increase the risk of participation in this study or interfere with the study results, or deemed unsuitable by the investigator.