To boost the adsorption-photocatalytic performances of biochar in wastewater treatment, biochar was modified with ternary metal oxides ZnO, Sm2O3, and MgO (ZnSm/Mg-biochar) via the one-step pyrolysis method. The optimal 2b-ZnSm/Mg-biochar with a Zn/Sm molar ratio of 1:1 exhibited the better removal activity of RhB in comparison with biochar, 2-Mg/biochar, and b-ZnSm/biochar. The removal efficiency and adsorption capacity of 2b-ZnSm/Mg-biochar was 99.46% and 979.22 mg g(-1) for RhB solution of 410 mg g(-1), respectively. The adsorption kinetics and isotherms of ZnSm/Mg-biochar composites were as described by second-order and Langmuir models, respectively. The abundant vacant sites at the junction interface of ZnO, Sm2O3, MgO, and biochar were favorable for the adsorption and photon capturing, achieving the efficient photo-conversion of RhB under the stimulated solar-light irradiation.