Objective To investigate the clinical effect of composite resin (CR) filling in the treatment of posterior teeth defect. Methods A total of 80 patients with posterior tooth defect admitted to our hospital from October 2019 to October 2021 were randomly divided into study group and control group, with 40 patients in each group. The control group was treated with glass ionomer cement (GIC) filling, and the study group was treated with composite resin (CR) filling. The clinical efficacy, chewing function and aesthetics, periodontal indexes [gingival index (GI), periotest values (PTV), plaque index (PLI)], oral health and adverse events were compared between the two groups. Results The total effective rate of treatment in the study group was 95.00%, which was higher than 75.00% in the control group (P <0.05). After treatment, the scores of bite force, masticatory efficiency and aesthetics in the study group were higher than those in the control group (P <0.05). After treatment, GI, PTV and PLI in the study group were lower than those in the control group (P <0.05). The scores of oral health impact (OHIP) in the study group were lower than those in the control group (P <0.05). The incidence of adverse events in the study group was 5.00%, which was lower than 27.50% in the control group (P <0.05). Conclusion In the treatment of patients with posterior teeth defects, CR can improve the clinical efficacy, improve the masticatory function and facial aesthetics, as well as improve the periodontal condition and oral health of patients, Meanwhile, it can reduce the incidence of adverse events.