Objective To investigate the clinical effect of CAD/CAM all-ceramic onlay in the repair of tooth defect after root canal therapy. Methods A total of 90 patients with dental defects admitted to our hospital from April 2019 to April 2022 were randomly divided into control group and study group, with 45 patients in each group. The control group was given traditional postcore all-ceramic crown restoration, and the study group was given CAD/CAM all-ceramic onlay restoration. The repair effect, periodontal status, language function, chewing function and satisfaction were compared between the two groups. Results The proportion of grade A of edge coloring, edge tightness, secondary caries and restoration wear in the study group was higher than that in the control group (P <0.05), while there was no significant difference in the proportion of grade A restorations falling off and fracture between the two groups (P >0.05). The GI, SBI and PLI of the two groups after repair were lower than those before repair, and those in the study group were lower than those in the control group (P <0.05). The scores of language function and chewing function in the two groups after repair were higher than those before repair, and those in the study group were higher than those in the control group (P <0.05). The satisfaction of the study group on the comfort of the restoration, the shape of the restoration, the color of the restoration, the time and number of visits were higher than those of the control group (P <0.05). Conclusion The application of CAD/CAM all-ceramic onlay in the repair of tooth defect after root canal therapy is effective, which can effectively improve the periodontal status, shape and color of the restoration. Meanwhile, it is conducive to improving the comfort of the restoration, reducing the time and frequency of treatment, and the repair effect is ideal, the patient satisfaction is high.