Objective: To investigate the effect of lithium disilicate ceramic onlays on masticatory function in endodontically treated posterior teeth. Methods: From January 2023 to January 2025, 84 patients with posterior tooth defects requiring coronal restoration after root canal therapy in our hospital were randomly divided into an onlay group (42 cases) and an all-ceramic crown group (42 cases). The onlay group received lithium disilicate ceramic onlay restoration, and the crown group received lithium disilicate all-ceramic crown restoration. Masticatory efficiency, complications and aesthetic satisfaction were evaluated. Results: At 12 months post-treatment, masticatory efficiency was higher than baseline in both groups, and was significantly higher in the onlay group (P<0.05). The complication rate (fracture, loss, secondary caries, marginal discoloration) was 2.38% in the onlay group and 19.05% in the crown group (P<0.05). Aesthetic satisfaction was 100.00% in the onlay group and 88.10% in the crown group (P<0.05). Conclusion: Lithium disilicate ceramic onlays improve masticatory function, reduce complications and enhance aesthetic satisfaction in endodontically treated posterior teeth.