Abstract: To address the issue of low strength in cemented fully tailings backfill and the susceptibility to pipe blockage during high−concentration filling, a mixed aggregate composed of waste rock and tailings as an alternative to single tailings aggregate for underground filling. In order to determine the optimal ratio of filling slurry, the physical and chemical properties of filling materials were characterized, and the grading of waste rock aggregate according to the Talbol theory. using the Box−Behnken central composite design and Response Surface Methodology (RSM) in the Design−Expert software, the effects of tailings−to−binder ratio, waste rock mass fraction, and slurry mass concentration on the mechanical strength of the filling body and the flowability of the filling slurry were systematically investigated. A ternary quadratic regression model was then established, taking the compressive strength and flowability of the filling slurry at different curing ages as response values, and its applicability was validated. The results revealed that the interaction between the tailings−to−binder ratio and slurry mass concentration significantly influenced the strength of the filling body, while the interaction between the waste rock mass fraction and slurry mass concentration prominently affected the flowability of the filling slurry. By setting the minimum filling cost as the optimization objective, a conditionally constrained model was established, yielding optimized filling slurry proportions of a 7∶1 tailings−to−binder ratio, 70% waste rock mass fraction, 84% slurry mass concentration, with a slurry consistency of 114 mm and a 28 day compressive strength of 5.13 MPa. These optimized proportions meet the requirements for both filling body strength and slurry flowability in mining applications.