Abstract: The theory of failure control of overlying rock in the roadway does not adequately address the challenges posed by the wide application of filling mining in coal mines, particularly in the combination of coal and backfill. This is due to the rapid development of engineering technology practice. Consequently, there is an increased occurrence of disasters, including stress concentration in the coal body, gangway failure, and dynamic outbursts of the coal body in the roadway. Understanding the degradation process and failure characteristics of the composite structure is crucial for effectively addressing and solving these disasters. In this paper, field testing and laboratory testing methods are employed to investigate the microstructural characteristics of the interface between the coal body and gangue, as well as the impact of the thickness of the backfill on the composite samples. This research is based on the practical experience that the backfill plays a significant role in controlling the stability of the coal body and the roadway. The research findings demonstrate a gradual decrease in the compressive strength of the composite body as the thickness of the backfill increases. At a thickness of 15 mm, the combination experiences a significant decrease in overall compressive strength, reaching 5.74 MPa. This value is 77.2% lower than that of the standard coal sample. The decrease in compressive strength is primarily attributed to the division of the coal sample by the backfill. This division leads to a decrease in the equivalent elastic modulus of the composite samples, compromising its integrity and bearing capacity. The failure mode and mechanical properties of the coal-rock combination are influenced by the placement of the backfill. In the process of uniaxial compression, the composite sample of coal and backfill undergoes four stages: firstly, when the composite samples begin to compress, the strain concentration starts from the contact interface between the coal sample edge and the backfill; secondly, as the bearing capacity increases, the strain concentration position shifts to the interface between the coal sample and the backfill; thirdly, before reaching the peak load, the strain concentration of the coal sample in the lower part of the composite samples increases and cracks expand; the fourth is that the composite samples fails after reaching its ultimate strength under loading. Moreover, the placement of the backfill significantly influences the failure mode and crack propagation in the composite samples. Cracks in the composite primarily concentrate at the upper and lower portions of the backfill when its thickness is 6 mm. At a thickness of 10 mm, the lower portion of the backfill experiences more apparent damage. Conversely, when the thickness reaches 15 mm, cracks predominantly occur at the midpoint of the composite samples. The research findings offer valuable theoretical insights into the control of deformation and instability in coal and rock masses in underground engineering. They are particularly significant in advancing the development of theories related to filling load bearing control and engineering design.