To explore the effectiveness of ceramic onlay on repairing young permanent molars with severe tooth defects. Methods A total of 76 patients with severe tooth defects of young permanent molars admitted to our hospital from January 2021 to July 2022 were selected as the research objects. According to the random number table method, they were divided into filling repair group and ceramic repair group, with 38 patients in each group. The filling repair group was filled with conventional resin, and the ceramic repair group was repaired with ceramic onlay. The tooth contact, bite force of affected teeth and contralateral teeth, repair effect and gingival health index were compared between the two groups. Results The immediate contact rate of the ceramic repair group was 89.47%, which was higher than 26.32% of the filling repair group (P <0.05). The bite force of the affected teeth and the contralateral teeth in the ceramic repair group was better than that in the filling repair group immediately after intervention and 3 and 6 months after intervention (P <0.05). The total effective rate of the ceramic repair group was 92.11%, which was higher than 78.95% of the filling repair group (P <0.05). There was no significant difference in gingival health index between the two groups (P >0.05). Conclusion The effect of ceramic onlay in the repair of young permanent molars with severe tooth defects is good, which can effectively improve the patient's tooth contact and improve the bite force of the affected teeth and the contralateral teeth.