Objective To compare the application effects of all-ceramic inlay restoration and direct resin filling restoration for tooth defect in adolescents. Methods A total of 40 patients with extensive molar defect treated in Tianshui Second People's Hospital from June 2024 to June 2025 were selected, and they were divided into the control group and the observation group according to different treatment schemes, with 20 patients in each group. Both groups received root canal therapy. The control group received directresin filling restoration, and the observation group received all-ceramic inlay restoration. The restoration effect, aesthetic effect and masticatory function were compared between the two groups. Results The excellent and good rates of retention, wear degree and marginal adaptation of restorations in the observation group were higher than those in the control group (P <0.05). There were no statistically significant differences in the excellent and good rates of marginal discoloration and tooth tissue integrity between the two groups (P >0.05). The scores of tooth morphology, color and aesthetics in the observation group at 12 months after restoration were higher than those in the control group (P <0.05). The occlusal force and masticatory efficiency in the observation group at 3 months after restoration were higher than those in the control group (P <0.05). Conclusion The overall effect of all-ceramic inlay for repairing tooth defect in adolescents is superior to direct resin filling. It can improve the retention, wear resistance and marginal adaptation of restorations, further optimize aesthetic effect, and effectively restore masticatory function.