This prospective, single-center, single-arm phase II clinical trial was designed to evaluate the efficacy and safety of bevacizumab plus nab-paclitaxel and S-1 as second-line treatment for patients with advanced biliary tract adenocarcinoma who experienced disease progression or intolerance after first-line systemic therapy. Participants received bevacizumab in combination with nab-paclitaxel and oral S-1 in 21-day treatment cycles until disease progression, unacceptable toxicity, death, withdrawal of consent, or other protocol-defined discontinuation criteria. The primary outcome was objective response rate assessed according to Response Evaluation Criteria in Solid Tumors version 1.1 (RECIST v1.1). Secondary outcomes included progression-free survival, disease control rate, duration of response, overall survival, quality of life, and safety. Exploratory analyses were conducted to investigate potential predictive biomarkers of treatment efficacy.