In order to reduce the cost of filling mining, slag powder-cement was used as a composite cementitious material, and the response surface method was used to obtain the optimal ratio of filling materials that could meet the strength requirements of a mine filling body. Firstly, through the strength test of filling body with different cement-sand ratios (slag powder-cement : tailings) and the slump test with different slurry mass concentrations, the ranges of slag powder dosage and slurry mass concentration were preliminarily determined. Then, based on RSM-BBD module, the strength regression model of slag-based cemented filling body at different ages was established, and the influences of slag powder dosage, cement-sand ratio and slurry mass concentration on the strength of cemented filling body were analyzed. The results show that the interactions of slag powder dosage and slurry mass concentration, slag powder dosage and cement-sand ratio, cement-sand ratio and slurry mass concentration have significant effects on the 3 d strength, 7 d strength and 28 d strength of the filling body, respectively. Finally, combined with the engineering requirements, the best ratio of filling materials was recommended. Mine practice shows that the use of slag-based cementitious materials instead of cement for mine filling can reduce the cost of filling cementitious materials by 12%−13%.