Objective To investigate the clinical effect of post-core crown and onlay restoration in patients with posterior tooth defect and its effect on gingival health. Methods A total of 94 patients with posterior tooth defect admitted to the Department of Stomatology, Rongcheng County People's Hospital from September 2022 to September 2023 were selected as the research objects. They were divided into control group and observation group by random number table method, with 47 patients in each group. The control group received post-core crown repair, and the observation group received onlay repair. The repair effect, chewing function, gingival status, quality of life and complications were compared between the two groups. Results The proportion of edge closure, complete restoration, color matching and smooth surface in the observation group was higher than that in the control group (P <0.05). The overall masticatory efficiency and bite force of the observation group were higher than those of the control group (P <0.05). The levels of GI, PLI and PD in the observation group were lower than those in the control group (P <0.05). The scores of each dimension of quality of life in the observation group were lower than those in the control group (P <0.05). The incidence of complications in the observation group was 2.13%, which was lower than 17.02% in the control group (P <0.05). Conclusion The repair effect of onlay restoration in patients with posterior tooth defect is better than that of post-core crown restoration, which can effectively improve the masticatory function and gingival status of patients, and is conducive to improving their quality of life. The incidence of complications is low and the safety is high.