Background: Patients with advanced esophageal squamous carcinoma (ESCC) who experience progression after first-line immunotherapy face a challenging treatment landscape. The potential of anti-vascular therapy combined with chemotherapy in the treatment of esophageal cancer is well-established. Mechanistically, anti-angiogenic agents can promote the normalization of tumor vasculature and facilitate the access of chemotherapeutic agents to tumor targets. Considering the compromised physical condition of advanced ESCC patients, there is a preference for more convenient oral medications. Therefore, we conducted this phase II study to assess the clinical efficacy and safety of combining fruquintinib with S-1 in treating advanced ESCC patients following first-line immunotherapy failure. Methods: This open-label, single-arm, phase II study consists of dose-finding and dose-expansion phases, enrolling advanced or metastatic ESCC patients after first-line immunotherapy failure. The dose-finding phase followed a 3+3 design. Patients received fruquintinib (3mg, 4mg, 5mg, d1-d14, q3w, respectively), in combination with S-1 (40mg, 50mg, 60mg, bid, d1-d14, q3w, based on body surface area (BSA)), until unacceptable toxicities, progressive disease, or death. The initial fruquintinib dose was 4 mg d1-d14, q3w, in combination with an appropriate S-1 dose. Additional patients were enrolled in the dose-expansion phase and received the maximum tolerated dose determined in the dose-finding phase. Results: As of August 1, 2023, seven patients were enrolled in the dose-finding phase. One patient withdrew consent during the DLT assessment period. Among the patients administered a 4mg dose of fruquintinib, one out of three achieved a confirmed partial response (PR) after 7 cycles of treatment. Additionally, two out of three patients at the 5mg dose level achieved PR after 2 cycles of treatment, resulting in an overall response rate (ORR) of 50% (3/6). The disease control rate (DCR) was 100% (6/6). The median progression-free survival (PFS) was observed to be 4.93 months (95% CI: 1.93 – 10.73), with overall survival (OS) at 10.73 months (95% CI: 2.1 – 10.73). In terms of safety, no dose-limiting toxicities (DLTs) were reported at 4mg dose level. While DLTs were observed in two out of the three patients at the 5mg dose level. The most common treatment-related adverse events (TRAE) across all grades were fatigue (5/7) and extremity pain (4/7). Grade 3 TRAE were identified in 5 patients, with hypertension (2/7), mucositis (2/7), and fatigue (2/7) being reported. No grade 4 or higher TRAEs were observed. Conclusions: Potential benefits were demonstrated when using fruquintinib in combination with S-1 for the treatment of ESCC patients after immunotherapy failure. The efficacy and safety of fruquintinib and S-1 will be further investigated in this trial later. Clinical trial information: NCT05636150.