To compare the application effect of composite resin and silver amalgam in the treatment of posterior teeth with proximal caries. Methods From June 2022 to June 2023, 120 patients with posterior teeth adjacent caries admitted to the Department of Stomatology, Yuyuantan Community Health Service Center, Haidian District, Beijing were randomly divided into control group and observation group, with 60 patients in each group. The control group was filled with composite resin, and the observation group was filled with silver amalgam. The filling success rate, filling material shedding rate, secondary caries incidence and comfort score were compared between the two groups. Results The filling success rate of the observation group was 96.67%, which was higher than 76.67% of the control group (P <0.05). The shedding rate of filling materials in the observation group was 1.67%, which was lower than 11.67% in the control group (P <0.05). The incidence of secondary caries in the observation group was 0, which was lower than 6.67% in the control group (P <0.05). The comfort score of the observation group was (7.40±1.09) scores, which was higher than (4.56±1.42) scores of the control group (P <0.05). Conclusion Compared with composite resin materials, silver amalgam filling posterior teeth adjacent caries can achieve higher filling success rate, and higher sealing. Meanwhile, it will not easy to fall off and secondary caries, and better comfort, which is worthy of clinical application.