The reason why a tooth kills the nerve and still hurts may be due to inflammation of the residual pulp, root fracture of the tooth, and mechanical irritation of the root tip. A tooth that kills the nerve and still hurts may be the result of a root fracture that occurred in the tooth during or after treatment. Inflammation of the residual pulp due to missing the root canal during treatment of a posterior tooth with multiple roots can also cause pain. Overstimulation of the root tip due to instruments during treatment can also lead to pain. 1. Inflammation of the residual pulp is caused by the root canal left behind during the treatment of multiple-rooted posterior teeth, or the root canal is too long and the extraction of the pulp is incomplete, resulting in pulp residue, the residual pulp will be painful during the treatment period, and the end of the treatment if the treatment is not perfect will also cause pain in the later stages, which will need to be retreated in order to alleviate the symptoms. 2. Root fracture often occurs in teeth that have undergone endodontic treatment, the teeth do not have pulp, there is no blood supply, it will be fragile under the action of external forces, such as eating hard objects will lead to tooth splitting, root fracture. During the treatment, the doctor will advise the patient not to eat hard objects, and after the treatment, crown repair will be recommended to prevent splitting. 3. During the treatment, the use of expanding needles, K-files, H-files, instruments beyond the root tip or pushing dentin debris out of the apical foramen can irritate the root tip and cause pain when preparing the root canal.