Abstract: Objective: To investigate the causes of root canal therapy failure by CBCT (cone beam computed tomography). Methods: 122 cases of root canal therapy failure in our hospital were selected. The causes of failure were analyzed by X-ray periapical film and CBCT. Results: in 122 cases of failure, 32 cases of root canal filling shortage (26.23%), 28 cases of root canal imprecision (22.95%), 23 cases of missing root canal (18.85%), 16 cases of combined periodontal-endodontic lesions (13.11%), 9 cases of root canal overcharge (7.38%), 7 cases of coronal leakage (5.74%), 6 cases of root fracture (4.92%), 1 cases of lateral canal perforation (0.82%). Conclusion: the main causes of root canal treatment failure are the incomplete cleaning of root canal and the inadequate obturation of root canal. The periodontal lesion is an important concomitant disease affecting the prognosis of root canal therapy. CBCT can provide a powerful basis for judging the cause of root canal therapy failure, and has an important guiding role in the follow-up treatment program.