Background: BTCs are a heterogeneous group of malignancies with a poor prognosis. Despite the improved outcome achieved by the addition of the anti-PD-L1 antibody durvalumab to standard chemotherapy, the benefits were modest and cannot satisfy the clinical demands adequately. This study aimed to evaluate the efficacy and safety of the combination therapy of Albumin paclitaxel, Durvalumab and Lenvatinib (ALLEN regimen) in unresectable BTCs. Methods: This single-arm, single-center, open-label, phase I/Ⅱ study enrolled patients with locally advanced or metastatic BTCs. Eligible patients with ECOG score 0-2 were treated with ALLEN regimen. Durvalumab (1000mg IV, q3w) in combination with albumin paclitaxel (180-220 mg/m² IV q3w) and lenvatinib (8mg PO QD) were given regularly until disease progression. The primary endpoint was objective response rate (ORR) evaluated by investigators according to RECIST 1.1. The secondary endpoints were disease control rate (DCR), progression-free survival (PFS) and treatment-related adverse events (TRAEs). Results: From September 2021 to September 2023, fourteen patients were enrolled including three patients who were failed to prior anti-PD-1 antibody treatment. The median follow-up was 6.8 months (95% CI 1.9, 10.9). The median age of the patients was 59.0 years, and male patients accounted for 35.7%. The ORR and DCR were 35.7% and 100%, respectively. The median PFS was 7.3 months (95% CI 4.9, 8.2). TRAEs of any grade occurred in 13 (92.9%) patients without grade 3 or higher. The most common TRAEs were alopecia (13/14, 92.9%), peripheral sensory neuropathy (10/14, 71.4%), decreased appetite and nausea (9/14, 64.3%), hypertension (6/14, 42.9%), rash (5/14, 35.7%), diarrhoea (4/14, 28.6%), and asthenia (4/14, 28.6%). Conclusions: The ALLEN regimen demonstrated encouraging anti-tumor activity with a tolerable safety profile in unresectable BTCs. The preliminary results were promising, which deserved further investigation in a larger population. Clinical trial information: NCT05327582.