Abstract: To study the influence of the height ratio on the collaborative load-bearing characteristics of the "roof-backfill" composite load-bearing structure in constructional backfill mining, rock-backfill composite specimens with height ratios ranging from 1:3 to 3:1 were prepared. Uniaxial compression tests were conducted, combined with digital image correlation (DIC) technology and acoustic emission (AE) monitoring. Meanwhile, a mechanical model was established to analyze the collaborative load-bearing characteristics and mechanisms.The results show that as the height ratio increases from 1:3 to 3:1, the strength of the rock-backfill composite increases from 5.6 to 11.71 MPa. When the height ratio is greater than 1:1, the strength of the rock-bcakfill composite is higher than that of the single backfill (8.96 MPa), accompanied by a synergistic strengthening effect.. Additionally, the failure mode transforms from splitting failure to tensile-shear mixed failure, and the failure location propagates from the partial backfill to the entire composite, thus achieving synergistic failure.Based on the interface bonding constraint force coefficient and Drucker-Prager strength criterion, the synergistic failure criterion is established as follows: The constraint effect at the interface weakens the sandstone strength while strengthening the backfill strength, thus contributing to the enhanced strength of the rock-backfill composite; when the height ratio increases to the critical value of 1:1.07, the strength of the rock-backfill composite exceeds that of the sandstone at the interface, enabling the failure of the backfill to propagate to the sandstone, thereby realizing the synergistic failure of the rock-backfill composite. The results can provide support for the selection of reasonable backfill positions in backfill mining.