Backfill–rock composite structures (BRCSs) are crucial for the stability of underground mining areas. However, during the mining and backfilling cycles, they are subjected to coupled dynamic-static loading. Herein, to systematically investigate the mechanical properties of BRCSs under in situ mining and filling stress loading, true triaxial dynamic-static tests were conducted. First, the effects of the depth, cement–tailings (C/T) ratio by mass, and interfacial angle (IA) on the composite strength, deformation characteristics, and failure modes were systematically investigated. Subsequently, the evolution of acoustic emission (AE) signal parameters during BRCS failure was analyzed. Finally, a damage constitutive model was established based on the AE energy analysis. With increasing depth, C/T ratio, and IA, the peak strength and elastic modulus of the BRCS exhibited an upward trend, and the strain during the loading–unloading disturbance stages correspondingly increased. At a C/T ratio of 1:8, the specimens exhibited a rock-dominated load-carrying capacity with distinct brittle failure. Conversely, at a C/T ratio of 1:4, the specimens demonstrated a coupled backfill–rock load-carrying capacity, exhibiting ductile failure in the shallow regions and a transition to brittle failure in the deeper zones. AE signals were concentrated during loading–unloading disturbance, plastic yielding, and failure stages. The dominant failure mode was tensile-shear composite fracture, with the proportion of shear cracks gradually increasing with depth. The damage evolution process of a BRCS can be divided into three stages: initial, accelerated, and ultimate failures. This study provides an important theoretical basis and practical guidance for optimizing C/T ratio and enhancing stability assessment in backfilled mine designs.
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