Objective To explore the effect of onlay restoration on restoration effect in patients with dental defect of posterior molars after root canal treatment. Methods A total of 160 patients with dental defect of posterior molars admitted to our hospital from October 2021 to December 2024 were selected and all received root canal treatment. According to the different restoration methods chosen by the patients, they were divided into two groups. 80 patients who received full crown restoration were included in the control group, and 80 patients who received onlay restoration were included in the observation group. The restoration effect, gingival index, inflammatory indexes and sati sfaction were compared between the two groups. Results The excellent and good restoration rate of the observation group (93.75%) was higher than that of the control group (81.25%) (P <0.05). The GI score of the observation group was better than that of the control group (P <0.05). The inflammatory indexes of the observation group were lower than those of the control group (P <0.05). The satisfaction score of the observation group was (89.93±2.07)scores, which was higher than (75.81±2.94) scores of the control group (P <0.05). Conclusion For patients with dental defect of posterior molars after root canal treatment, onlay restoration has a better restoration effect. It can not only improve the tooth function but also reduce the inflammatory indexes and increase the satisfaction of patients, which is worthy of clinical promotion.