The separation of overburden strata in coal mining directly affects surface subsidence, the ecological environment, and mining safety. Backfilling technology is currently the primary solution to address these issues. This study examines the rheological performance of a coal gangue and fly ash mixed slurry as a filling material under different raw material proportions and injection pressures, and predicts its diffusion distance using a theoretical model. Orthogonal experiments were conducted to evaluate the influences of solid volume concentration, coal gangue particle size, and fly ash-to-coal gangue mass ratio on the density, viscosity, and water bleeding rate of the slurry. Results demonstrated that solid volume concentration had the most significant influence on density and viscosity, followed by coal gangue proportion and particle size. Increasing coal gangue content elevated density and viscosity due to higher interparticle friction, while finer particles reduced viscosity by 30–40%. The introduction of a polycarboxylate superplasticizer achieved a 45% viscosity reduction with an optimal dosage of 0.3 wt%, by dispersing particles and enhancing the availability of free water compared to sulfamic acid. Injection pressure accelerated water bleeding rates by 20–35%, while finer coal gangue particles prolonged bleeding time. A power-law fluid fracture grouting diffusion model predicted that higher injection pressures (0.1–0.4 MPa) and fracture widths (0.4–1.0 mm) linearly increased diffusion distance, whereas steeper fracture angles (5–20°) enhanced the spread range. This study provides a broad perspective for designing cost-effective, environmentally stable grouting systems using coal-based waste, balancing injectability and long-term performance in mining applications.