<p>To mitigate strata movement and surface subsidence commonly induced during coal extraction beneath surface structures, this study targets the safe mining of coal resources under buildings, railways and water bodies (“three-under”conditions) and the control of mining-induced damage. By optimizing the mix design of backfilling materials, the work aims to enhance resource recovery while reducing environmental impacts associated with coal gangue accumulation. An orthogonal experimental design was employed to optimize the proportions of an early-strength agent, a retarder, and a suspending agent, and to quantify the relative contributions of each component to the compressive strength of the cured backfill at 7 and 28 d. Guided by the orthogonal results, a compound admixture was formulated, and single-factor experiments were conducted to evaluate the effect of admixture dosage on backfill strength. As the admixture dosage increased, the 3, 7, and 28 d compressive strengths followed an increase-then-decrease trend. Backfill specimens containing the admixture exhibited failure modes indicative of superior structural integrity, with markedly enhanced interfacial bonding. SEM-EDS analysis showed that the admixture promoted hydration, provided an alkaline environment conducive to the pozzolanic reaction of fly ash, and refined the pore structure. By 28 d, Ca(OH)₂ formed at early ages accelerated the pozzolanic reaction, enabling synergistic fly ash-cement interactions that generated abundant hydration products. Moreover, Mg²⁺ was not detected in the admixed group, thereby avoiding deleterious crystallization-induced expansion associated with Mg(OH)₂. Through the combined mechanisms influence of pore refinement, interfacial strengthening, and suppression of Mg²⁺ release, the admixture enhanced backfill strength and supported long-term stability.</p>