Objective To compare the application effects of all-ceramic onlay and all-ceramic crown restorations after root canal therapy of premolars. Methods A total of 82 patients who underwent root canal therapy of premolars and were treated in Suzhou Stomatological Hospital from January 2020 to December 2022 were selected. According to different restoration methods, they were divided into group A and group B, with 41 patients in each group and 48 affected teeth in each group. Group A was given allceramic onlay restoration, and group B was given all-ceramic crown restoration. The restoration effects (short-term and long-term), masticatory function, adverse reaction rate and oral health-related quality of life were compared between the two groups. Results There was no significant difference in the total effective rate of short-term restoration between the two groups (P>0.05). The total effective rate of long-term restoration in group A (95.83%) was higher than that in group B (83.33%) (P<0.05). The masticatory efficiency and occlusal force of group A after restoration were higher than those of group B (P<0.05). The incidence of adverse reactions in group A (2.44%) was lower than that in group B (19.51%) (P<0.05). The OHIP-14 score of group A at 2 years after restoration was lower than that of group B (P<0.05). Conclusion The short-term effects of all-ceramic crown restoration and all-ceramic onlay restoration after root canal therapy of premolars are comparable, but the long-term effect of all-ceramic onlay restoration is more ideal, with better safety. It can effectively improve masticatory function and is beneficial to enhancing patients' oral health-related quality of life.