Objective To observe the effect of root internal retainer combined with crown external retainer on the repair of most posterior tooth defect. Methods A total of 120 patients with most posterior tooth defect who underwent repair treatment in our hospital from January 2018 to April 2023 were selected as the research objects. They were divided into reference group and study group by random number table method, with 60 patients in each group. The reference group was repaired with extracoronal retainer, and the study group was repaired with intracoronal retainer combined with extracoronal retainer. The clinical effect, periodontal health score, pain (VAS) score and tooth surface defect score were compared between the two groups. Results The total effective rate of treatment in the study group was 96.67%, which was higher than 83.33% in the reference group (P <0.05). The periodontal health score of the two groups after treatment was lower than that before treatment, and that of the study group was lower than that of the reference group (P <0.05). The score of tooth surface defect in the two groups after treatment was lower than that before treatment, and that of the study group was lower than the reference group (P <0.05). The VAS score of the two groups after 6 and 24 h of treatment was lower than that before treatment, and that of the study group was lower than that of the reference group (P <0.05). Conclusion The effect of root internal retainer combined with crown external retainer in repairing most posterior tooth defect is exact, which can reduce the pain of patients, improve periodontal health and reduce the score of tooth defect. It is an effective combined treatment plan.