This is a prospective, single-center, randomized controlled, phase II clinical trial. The study aims to enroll 48 patients with resectable, locally advanced gastroesophageal junction adenocarcinoma who have not received any treatment. After obtaining informed consent and meeting the inclusion/exclusion criteria, patients were randomly assigned preoperatively in a 1:2 ratio: Immunomodulation group (n=32): 3 cycles of slulimab combined with SOX combined with radiotherapy and 9 weeks of neoadjuvant thymosin; Radiochemoimmunotherapy group (n=16): 3 cycles of slulimab combined with SOX combined with radiotherapy; Radiotherapy was initiated 2-5 days after the start of the second cycle of immunochemotherapy. A 5-10 mm extravasation was made from the endoscopically marked tumor boundary and adjacent metastatic lymph nodes to form a central tumor volume (CTV), and a 5-10 mm extravasation was made to form a partial tumor volume (PTV). The planned PTV treatment time was 44 Gy/22 fractions per minute (F), 5 fractions per week (F/W). After neoadjuvant therapy, the efficacy of the therapy and the feasibility of radical D2 resection are assessed through imaging examinations. Efficacy evaluation is performed within 2 weeks of the completion of neoadjuvant therapy, and radical gastrectomy is performed within 4-6 weeks. Postoperative treatment is determined jointly by the clinician and the patient based on actual clinical practice. The primary endpoint is the safety of neoadjuvant therapy: the incidence of ≥ grade 3 treatment-related adverse events (TRAEs) during the perioperative period (from the start of neoadjuvant therapy to one month postoperatively). Safety assessment: Safety assessments are performed after each cycle of neoadjuvant therapy and 30 days postoperatively. Event follow-up: Follow-up events are then conducted every 3 months for the first year postoperatively, and every 6 months for 1-2 years, up to 2 years postoperatively.