In view of the problems of large stacking of solid waste caused by roof caving method in coal seam mining and imbalance between mining and filling in long-wall cemented backfilling, the No.31 ming area of Chahasu Coal Mine in Inner Monogolia uses continuous mining and filling roadway cemented backfilling mining technology, adopting the “mining at intervals of three” mining method and the “three with high strength filling and one with low strength filling” filling mode. The filling material ratio has an important influence on the filling cost and effect. In order to solve the problems of high cement consumption and less absorption of fly ash utilization in the original cemented filling materials, the optimal particle size grading coefficient n of gangue was determined based on Talbot gradation theory as 0.5, and material ratio was optimized using the Response Surface Method (RSM). Based on Box–Behnken Design (RSM-BBD) of 13 groups of tests, a regression models of the single-axis compressive strength of the backfilling body at 3, 7 and 28 days with the cement mass fraction X₁, fly ash frection X₂ and solid mass fraction X₃, and the interaction of the three factors (X₁X₂, X₁X₃, and X₂X₃) was establish ed. The P values of the models were all less than 10⁻⁴, which indicates that the model has strong reliability. The test results show that not only does the single factor has a significant effect on the strength, but the interaction of factors also has a certain impact on the strength. Specifically, the strength of backfill increases with the increase of X₁ and X₃, but increases first and then decreases with the increase of X₂. Besides, X₁ has the most significant effect in the early stage, X₃ has the most significant effect in the later stage, while the effect of X₂ on the strength gradually becomes significant as the age period increases. X₂X₃ influences the 7-day strength of backfill significantly, X₁X₂ influences the 28-day strength significantly, and X₁X₃ has a significant effect on the strength at 3, 7 and 28 days. In the interaction effect of X₁X₃ on the strength of backfill at 7 and 28 days, the content and mass concentration of cement promote each other, capable of increasing the strength, while in the interaction, the increase of one factor will have an inhibitory effect of the other. Base on RSM, the reasonable ratio was determined as X₃ at 78%‒79%, X₂ at 19%‒20%; X₁ at 9%‒10% for high strength filling, and X₁at 8% for low strength filling. The No.303 working face of Chahasu Coal Mine mined a total of 1.68×10⁵ tons of coal, and consumed 1.622×10⁵ tons of gangue , with an increase of fly ash utilization and an decrease of cement consumption of more than 1.0×10⁴ t, greatly saving of fly ash treatment and cement cost, bringing good environmental and economic benefits.