Apical periodontitis in a filled tooth can be treated with a root canal after removing the material of the filling. Apical periodontitis is caused by bacteria in the pulp cavity infecting the pulp and spreading to the alveolar bone through the apical foramen, resulting in destruction of the alveolar bone. Failure to pay attention to oral hygiene and secondary caries after filling can lead to apical periodontitis, which manifests itself as occlusal pain, upper and lower teeth not daring to bite, tooth floating sensation, and loosening of teeth. When apical periodontitis occurs in a filled tooth, it is necessary to remove the filling material and then perform root canal therapy. The pulp chamber is opened and thoroughly sterilized, and then the root canal is filled to seal the root canal of the tooth to avoid re-infection. After the apical periodontitis recovers, the tooth can be filled again. When apical periodontitis occurs in a filled tooth, you should seek standardized treatment as soon as possible to avoid aggravating the condition and causing adverse consequences.