Objective To analyze the effect of onlay restoration on oral health and aesthetics in patients with posterior tooth defect undergoing root canal treatment. Methods A total of 90 patients with posterior tooth defect undergoing root canal treatment in the Oral Medicine Center (Oral Branch) of Suining Central Hospital from October 2022 to October 2023 were selected. They were divided into the control group and the observation group according to the random number table method, with 45 patients in each group. The control group was given conventional restoration, and the observation group was given onlay restoration. The oral health status, aesthetics, complication rate, inflammatory factor levels and satisfaction were compared between the two groups. Results After restoration, the scores of PLI, SBI and GI in the observation group were lower than those in the control group (P <0.05). After restoration, the score of aesthetics in the observation group was higher than that in the control group (P <0.05). The incidence of complications in the observation group was lower than that in the control group (P <0.05). After restoration, the levels of TNF-α, IL-1β and IL-4 in the observation group were lower than those in the control group (P <0.05). The satisfaction in the observation group was higher than that in the control group (P <0.05). Conclusion For patients with posterior tooth defect undergoing root canal treatment, the application of onlay restoration can effectively improve their oral health and aesthetics, reduce the levels of inflammatory factors and the incidence of complications, and the patients' satisfaction is high.