Abstract: In constructional backfill mining, variations in the heights of the roof and coal seam lead to the formation of roof–backfill composite load-bearing structures with different height ratios between the backfill columns and roof at different backfilling positions. Such dimensional variations lead to performance differences between the roof and backfill, which in turn alter the load–bearing and failure characteristics of the composite structures, ultimately impacting the effectiveness of constructional backfill mining. To address this issue, rock–backfill composite specimens with height ratios ranging from 1∶3 to 3∶1 were prepared. Uniaxial compression tests were conducted, and the failure characteristics were monitored by combining digital image correlation (DIC) technology with acoustic emission (AE) monitoring. A mechanical model was established to analyze the synergistic-bearing characteristics and their underlying mechanisms. The results showed that as the height ratio increased from 1∶3 to 3∶1, the strength of the rock–backfill composite increased from 5.60 to 11.71 MPa. When the height ratio exceeded 1∶1, the strength of the composite was higher than that of the single cemented gangue backfill specimen (8.96 MPa), exhibiting a synergistic strength enhancement effect. The failure mode transformed from splitting failure to tensile-shear mixed failure, and the failure region propagated from the local backfill region to the entire composite, thereby achieving synergistic failure. Analysis of AE data revealed that the failure mode transitioned from the sole failure of the backfill to the combined failure of the sandstone and backfill owing to the variation in the height ratio of the specimens. When the height ratio was less than 1∶1, no synergistic failure occurred, and the cumulative AE ringing count increased gradually; when the height ratio was greater than or equal to 1∶1, synergistic failure occurred, and the failure of the sandstone caused a sudden surge in the cumulative AE ringing count. The average failure intensity of the specimens increased with the height ratio; however, the growth rate decreased when the height ratio exceeded 2∶1. From the AE location points and DIC strain contours, it was found that stress concentration occurred easily at the interface when the height ratio was less than 1∶1, leading to premature failure of the composite. Moreover, the failure position of the specimens shifted from the backfill to the sandstone part as the height ratio increased. Based on the interface bonding constraint force coefficient and Drucker–Prager strength criterion, a synergistic failure criterion was established. The interface constraint effect reduces the strength of the sandstone while increasing that of the backfill, thereby enhancing the overall strength of the composite. The critical height ratio for synergistic failure was calculated to be 1∶1.07. When the height ratio exceeded this value, the strength of the composite was higher than that of sandstone at the interface, and the failure in the backfill propagated into the sandstone. The bearing capacity of the rock–backfill combined body increased with the height ratio, and all specimens exhibited collaborative failure. This behavior enables full utilization of the surrounding rock’s bearing capacity, thereby satisfying the requirements of constructional backfill mining. These results support the selection of reasonable backfilling positions for constructional backfill mining. In specific engineering practices, it is necessary to comprehensively consider potential influencing factors such as interface parameters and geological conditions. The critical value should be appropriately modified and optimized according to specific engineering conditions, and the rationality of the layout of key positions of the roof-backfill composite load-bearing structure should be evaluated for constructional backfill mining. This enables the structure to adapt precisely to engineering practices and enhances the practical applicability of the research findings.