Study design: Prospective, single-arm, single-center phase II clinical study; Primary endpoint: Conversion rate; Secondary endpoints: Safety, disease control rate, disease-free survival, and overall survival; Main characteristics of enrolled patients: Patients with initially unresectable biliary tract cancer; Interventions: Combination of Gemcitabine, Oxaliplatin, Sintilimab and Bevacizumab; Sample size: 34 patients; Treatment until: 1. successfully conversed to resectable disease 2. progressed disease 3. intolerable toxicity 4. patient requests withdrawal; Research process: In this study, patients who met the inclusion criteria were evaluated at the end of every 3 weeks of treatment, up to surgical treatment or disease progression; Safety evaluation: Evaluate adverse reactions according to CTCAE 4.0; Follow up: 12 months after the last case was enrolled.

Inclusion Criteria:

1. Age ≥18 and ≤80 years;

2. ECOG 0~1;

3. Histologically or cytologically confirmed carcinoma of the bile duct or gallbladder;

4. Imaging assessment of disease stage III/IVA/any TN1M0*;

5. The main organs have good functions and the examination indexes meet the following requirements:

6. Blood routine test:

   Hemoglobin ≥90 g/L (no blood transfusion within 14 days); Neutrophils count ≥1.5×10^9/L; Platelet count ≥80×10^9/L;

7. Biochemical tests:

   Total bilirubin ≤2×ULN (upper limit of normal value); Blood alanine aminotransferase (ALT) or blood aspartate aminotransferase (AST) ≤ 2.5×ULN; Endogenous creatinine clearance rate ≥ 50 mL /min (Cockcroft-Gault formula);

8. Voluntarily signed the informed consent;

9. Good compliance and family members are willing to cooperate with follow-up.

Exclusion Criteria:

1. Other uncured malignancies;
2. Pregnant or lactating women, if the subject becomes pregnant during the study period, should withdraw from the clinical trial;
3. Previous anti-tumor therapy for the disease in this study;
4. Participated in other drug clinical trials within one month;
5. Patients with known history of other systemic serious diseases before screening;
6. Long-term unhealed wounds or incomplete healed fractures;
7. Have a history of organ transplantation;
8. Abnormal blood coagulation, with bleeding tendency (14 days before randomization must meet: INR within the normal range without the use of anticoagulants); Patients treated with anticoagulants or vitamin K antagonists such as warfarin, heparin or their analogs; The use of low-dose warfarin (1 mg orally, once daily) or low-dose aspirin (not more than 100 mg daily) for prophylactic purposes is permitted, provided that INR is less than 1.5;
9. The incidence of arterial/venous thrombosis events in the previous year, such as cerebrovascular accident (including temporary ischemic attack), deep venous thrombosis and pulmonary embolism, was screened;
10. People with a history of psychotropic substance abuse and unable to get rid of it or with mental disorders; Have a history of immunodeficiency, or other acquired or congenital immunodeficiency diseases, or have a history of organ transplantation;

12. Concomitant diseases that, in the Investigator's judgment, seriously endanger patient safety or affect patient completion of the study.