Objective To explore the effect of crown lengthening combined with fiber post-core crown restoration on tooth defect and its influence on periodontal index. Methods A total of 60 patients with tooth defect admitted to the First People's Hospital of Xining from January 2021 to January 2023 were selected as the research objects. They were divided into control group and observation group by random number table method, with 30 patients in each group. The control group was treated with gingival resection combined with fiber post-core crown restoration, and the observation group was treated with crown lengthening combined with fiber post-core crown restoration. The clinical efficacy, periodontal index, inflammatory factor level and adverse reactions were compared between the two groups. Results The total effective rate of treatment in the observation group was 96.67%, which was higher than 80.00% in the control group (P <0.05). The periodontal pocket depth, gingival index, bleeding index and plaque index of the observation group were better than those of the control group (P <0.05). The levels of IL-1β and IL-6 in the observation group were lower than those in the control group (P <0.05). The incidence of adverse reactions in the observation group was 0, which was lower than 13.33% in the control group (P <0.05). Conclusion Crown lengthening combined with fiber post-core crown restoration is effective in the treatment of tooth defects, which can effectively improve the periodontal index of patients, reduce the level of inflammatory factors and reduce the incidence of adverse reactions. It is worthy of clinical application.