Background: Biliary tract cancer (BTC) is a relatively rare but highly aggressive malignancy. Gemcitabine + cisplatin ±durvalumab is the standard first-line treatment for advanced BTC. But in second-line setting, there is no satisfactory regimen for most patients without specific genetic mutations. This prospective, single-arm, phase 2 study is conducted to investigate the efficacy and safety of second-line nab-paclitaxel combined with sintilimab in advanced BTC patients. Methods: Eligible patients are histologically confirmed local advanced or metastatic adenocarcinoma or cholangiocellular carcinoma in biliary tract; aged from 18 to 75 years; with a performance status and adequate organ function; with documented disease progression after prior gemcitabine or fluorouracil-based systemic chemotherapy; not received taxol drugs or immune checkpoint inhibitors treatment. Enrolled patients will receive nab-paclitaxel 125mg/m² on day 1, 8 plus sintilimab 200mg on day 1, administered intravenously every three weeks. The primary end point is objective response rate (ORR). The secondary end points are progression free survival (PFS), overall survival (OS) and adverse reactions. Next-generation sequencing (NGS) is voluntary before treatment. Results: Up to September 2022, 24 BTC patients have been enrolled, including 22 patients available for efficacy evaluation. The ORR is 27.3% (6/22), including 1 complete response (CR) and 5 partial response (PR). Disease control rate is 63.6% (14/22). And median PFS is 5.63 months (95% CI: 1.92-9.35 months). Median OS from enrollment to death of any cause is 12.60 months (95%CI: 7.84-17.36 months). The frequent grade 3 drug-related adverse events are leukopenia (16.7%), anemia (12.5%), neutropenia (8.3%), peripheral neuropathy (8.3%). There is no grade 4 drug-related adverse event occurred. One patient has experienced grade 3 immunological colitis. 11 participants have received NGS testing, with 9 ones are available for efficacy evaluation. Among the 9 participants, 5 patients harbor TP53 mutation [2 evaluated as PR, 2 SD (stable disease) and 1 PD (progressive disease)]; 3 patients harbor genetic mutations associated with DNA damage response pathway (2 evaluated as PR and 1 PD); and 1 patient is in status of microsatellite instability-high (MSI-H) evaluated as PR. Conclusions: The preliminary results of NapaSinti trial propose a brand new and effective regimen as second-line treatment for advanced biliary tract cancer. NGS may become a hopeful tool for efficacy prediction. Clinical trial information: ChiCTR2100052118.