Background: PD-1 inhibitor combined with chemotherapy is one of the standard regimens for first line gastric. The addition of anti-vascularization agents has been reported to help improve the overall survival (OS) in GC/GEJC patients (pts) through vascular-normalization in the microenvironment. In this phase I/IIa study, we aim to explore the safety and efficacy of tislelizumab combined with anlotinib and XELOX (TAXE regimen) as first line treatment for advanced GC/GEJC. Methods: This prospective, multicenter, phase I/IIa study planned to enroll 9 patients in phase I and 57 patients in Phase IIa. The aim of this Phase 1 study was to determine dose-limiting toxicities (DLTs), maximum tolerated dose (MTD) and safety of 8 mg, 10 mg or 12 mg anlotinib in combination with XELOX and tislelizumab. The maximum tolerable dosage (MTD) would be administered to all pts in phase II. The primary and objective response rate(ORR). Secondary endpoints included disease control rate (DCR), duration of response (DoR), progression-free survival (PFS), OS, and safety. The exploring endpoint was to evaluate the association between the efficacy and the expression of biomarkers including PD-L1, microsatellite instability profile, and genome stability (GS). Results: From march 2021 to August 2022, 38 pts were enrolled (9 pts in phase 1). Phase 1 was completed without dosage limiting toxicities (DLTs). 12mg anlotinib was recommended for phase 2. Thirty two (84.21%) pts had at least one treatment-related adverse events (TRAEs), and 7 patients (18.42%) had at least one grade 3/4 TRAEs. Anemia (15 pts, 39.47%), leukopenia (12 pts, 31.58%) and fatigue (10 pts, 26.32%) were the most common TRAEs. Two pts achieved complete response (CR), 20 pts achieved partial response (PR), and 7 pts achieved stable disease (SD). The ORR was 75.86% (95% CI: 57.89%-87.78%), and the DCR was 100%. Conclusions: The preliminary results of this Phase I/IIa trial by using tislelizumab combined with anlotinib and XELOX had shown great potential in improving the response rate in advanced GC/GEJC patients with manageable toxicity. Clinical trial information: NCT04963088.