To investigate the effect of polymerized porcelain and composite resin onlay restoration in root canal therapy and its effect on the fracture resistance of premolars. Methods From March 2021 to May 2022, 80 patients who underwent root canal treatment in Zibo 148th Hospital were selected as the research objects. They were divided into observation group and reference group by random number table method, with 40 patients in each group. The reference group was repaired with composite resin onlays, and the observation group was repaired with polymer ceramic onlays. The repair effect, premolar fracture resistance and repair satisfaction were compared between the two groups. Results The repair success rate of the observation group was 97.50%, which was higher than 80.00% of the reference group (P <0.05). After treatment, the fracture resistance of premolars in the observation group was better than that in the reference group (P <0.05). The repair satisfaction of the observation group was 97.50%, which was higher than 85.00% of the reference group (P <0.05). Conclusion Polymer ceramic onlay restoration can effectively improve the fracture resistance of premolars after root canal treatment, and its repair effect is better than that of composite resin onlay restoration, and patients have higher satisfaction with the repair.