Abstract: In coal mine structural backfilling mining, arranging backfill at key goaf positions forms a "Backfill-Surrounding rock" synergistic bearing structure, effectively controlling surface subsidence and improving mining efficiency. Insufficient interface strength causes sliding and shear failure between backfill and surrounding rock, undermining “immediate roof-backfill-immediate floor” composites stability. To explore interface strength’s effect on the synergistic bearing and failure characteristics of the “immediate roof-backfill-immediate floor” composites, uniaxial compression tests were conducted on rock-backfill-rock composites with interface strength of 1.43～30.87 MPa, obtaining the variation laws of composites strength and elastic modulus with interface strength. Combined with DIC digital image correlation and acoustic emission（AE）, the failure characteristics were accurately analyzed, and the synergistic failure mechanism was revealed by integrating interface mechanical properties and energy accumulation-loss characteristics. The test results show that composites strength and elastic modulus increase with interface strength. At 1.43 MPa, propagate laterally along the interface, and are mainly distributed in the backfill; At 5.69 MPa, high-energy AE events appear in sandstone, with AE energy and cumulative energy increase in order of magnitude; backfill cracks extend to sandstone through the interface after penetration, realizing synergistic failure, and this interface strength is lower than the backfill specimen strength 11.30 MPa. As interface strength increases from 1.43 to 30.87 MPa, dominant failure cracks transform from tensile to shear. Interface mechanical analysis indicates that above the critical threshold of 5.15 MPa, the maximum interface shear stress（5.61～17.70 MPa）is lower than the corresponding composites interface strength（5.69～30.87 MPa）, avoiding sliding failure. The maximum interface compressive stress 11.41～12.26 MPa exceeds backfill specimen strength, restraining backfill damage; composites strength is dominated by backfill strength and assisted by interface shear stress, enabling synergistic bearing. Drucker-Prager strength criterion analysis shows that interface constraint weakens sandstone strength but enhances backfill strength; higher interface strength helps energy released by backfill failure reach sandstone’s energy storage limit. Energy analysis reveals that at 5.69～30.87 MPa, the composite’s energy storage limit（20.22～26.31 kJ·m⁻³）and peak dissipation ratio（0.20～0.26）both exceed those of the backfill specimen 20.15 kJ·m⁻³ and 0.17, with higher crack penetration degree and synergistic failure. In conclusion, spaced backfill arrangement in structural backfilling mining requires interface strength exceeding the critical threshold to give full play to the bearing capacity of roof and floor strata. The results are of great significance for improving the stability of the “immediate roof-backfill-immediate floor” composites bearing structure.