<p>The goaf backfilling of coal gangue can effectively control the deformation and failure of overburden and suppress the development of water-conducting fracture zones. However, different particle size gradations of coal gangue lead to varying compressive deformations of backfill materials under the action of overburden stress, indirectly affecting the deformation, failure, and permeability of the overburden after the goaf backfilling of coal gangue. To this end, coal gangue backfill materials with five different particle size gradations were prepared, the compressive deformation characteristics of these materials under stress were tested in this study. Additionally, a simulation method for coal gangue backfill materials based on the experimental results was proposed and embedded into FLAC^3D numerical software, and a numerical model for goaf backfilling of coal gangue under the aquifer was established. Finally, the deformation and failure characteristics of overburden after goaf backfilling of coal gangue were analyzed under different backfill ratios, and the distribution law of pore water pressure in overburden was revealed. The results show that as the backfill ratio increases, the range of overburden deformation and failure declines and the pore water pressure gradually decreases. When the backfill ratio is greater than 70%, the overburden deformation and failure can be effectively controlled, reducing the risk of water inrush in the working face. This research provides theoretical support for the prevention and control of mining-induced water inrush hazards under water bodies.</p>