To explore the effect of fiber post restoration on large area defect of posterior teeth. Methods A total of 42 patients with large area defect of posterior teeth admitted to Midu County People's Hospital from January 2021 to December 2022 were selected as the research objects. According to different treatment methods, they were divided into control group and observation group, with 21 patients in each group. The control group was repaired with cast metal post, and the observation group was repaired with fiber post. The clinical efficacy, aesthetic degree and masticatory function of the two groups were compared. Results The total effective rate of treatment in the observation group was 95.24%, which was higher than 71.43% in the control group, and the difference was statistically significant (P <0.05). After treatment, the PES score and WES score of the observation group were higher than those of the control group, and the differences were statistically significant (P <0.05). After treatment, the bite force and masticatory efficiency of the observation group were higher than those of the control group, and the differences were statistically significant (P <0.05). Conclusion The effect of fiber post restoration in patients with large area defect of posterior teeth is ideal, which can not only effectively improve the beauty of teeth, but also improve the masticatory function of patients.