<p>The stability of rock–backfill systems critically impacts the safety and efficiency of mining operations. However, the failure characteristics of the load-bearing synergy between rock and backfill under dynamic loads are unclear, and the interrelation between rock–backfill composites (RBC), rocks, and backfills has not been considered. Therefore, split Hopkinson pressure bar (SHPB) laboratory tests and a coupled simulation using the Finite Difference Method (FDM) and Discrete Element Method (DEM) were employed to investigate the dynamic mechanical behavior and fracture characteristics of rock, backfills, and RBC. The dynamic stress–strain curves of the RBC can be divided into four stages: linear elasticity, plastic deformation, crack coalescence, and final failure. Rock dominates the strength of the RBC, and this dominant trend becomes more pronounced as the cement-tailings ratio (CTR) of the backfill decreases. Failure is predominantly governed by the weaker side of the contact interface, and the layered interface plays a 'separating' role in backfill failure on both sides. The failure modes of the RBC are classified as large fragmented bodies, pulverized fractures, and coupled failures of rock and backfill. Instability strength models of the RBC under different load-bearing modes were constructed, with the theoretical strength values corresponding to the experimental and simulation results. Additionally, the mechanism of the layered interface effect was elucidated, and the strength rule of the backfill in different regions was confirmed. The research results are of instructive significance for the monitoring of rocks and backfill and evaluating instability. </p>