When a fistula develops in a tooth after a filling, it is important to identify the source of the fistula, whether it is of apical or periodontal origin. Usually with a fistula of periodontal origin, the tooth will have deep periodontal pockets, loss of attachment, and moderate to severe resorption of the alveolar bone in the apical fragment, etc. The case requires consultation with a periodontist for periodontal inflammation. If a periodontal source is ruled out and an apical source is favored, a dental cement tip is usually inserted at the fistula as a diagnostic wire and an apical radiograph is taken to determine the source of the fistula. If it originates from chronic inflammation of the root tip, the tooth has developed periapical disease, occlusal trauma or combined periodontal and pulpal lesions, the situation requires root canal treatment to eliminate the inflammation of the root tip; combined periodontal and pulpal lesions require combined periodontal and pulpal treatment; if the root is in other teeth, the other teeth are required to be treated accordingly at the same time. Therefore, patients should not be alarmed or speculate about fistulas in teeth after fillings, but seek timely medical attention.