Abstract: High-content CaCO₃-filled unsaturated polyester resin (UPR) composites hold significant application value in construction and mechanical processing fields. The regulation of interfacial bond strength is a core issue for enhancing the mechanical properties of artificial stone, novel resin mortars, and machine tool load-bearing materials. Addressing the challenge of quantitative multiscale failure mechanism analysis in high-filler CaCO₃/UPR composites, this study established a four-phase discrete element model (DEM) incorporating aggregate, resin mortar, interface, and void phases through uniaxial compression tests. The effects of gradient variations in interfacial strength on stress-strain behavior, energy evolution, force chain development, failure modes, and internal particle displacement fields were systematically investigated. Results revealed that the damage evolution of high-filler CaCO₃/UPR composites follows a four-stage pattern: elastic phase, stable crack propagation phase, unstable crack propagation phase, and post-peak softening phase. When the interfacial strength was 0.2 σ_m, the strain energy reached 4.02 kJ, exceeding the crack propagation threshold of 1.95 kJ, leading to structural instability induced by multiphase separation. Increasing interfacial bond strength from 0.2 σ_m to 1.0 σ_m enhanced compressive strength by 31.5%, elevated crack initiation stress by 445.3%, and increased strain energy at peak stress from 73 kJ to 98 kJ, confirming that interface strengthening improves energy absorption and deformation resistance. Failure stage transitions were governed by interfacial strength thresholds: At interfacial strengths ≤0.6 σ_m, cracks predominantly propagated along aggregate interfaces, forming shear failure patterns with continuous force chain fractures, ultimately causing crack-blocking failure. When interfacial strength approached or exceeded 0.8 σ_m, crack nucleation shifted to matrix-interface composite regions, accompanied by reduced crack density, altered propagation paths, and enhanced particle displacement symmetry. Interface optimization enhances material performance through dual mechanisms: delaying friction/slip-induced dissipation processes to elevate crack initiation stress, and redirecting crack paths to transition failure modes from interfacial debonding to matrix-dominated fracture. This work provides theoretical insights for optimizing organic-inorganic interfaces in high-filler CaCO₃/UPR composites.