Abstract: Disposal and utilization of coal gangue is one of the main problems faced by coal mining in China, and the injection of coal gangue into the form of slurry can effectively solve the problems of ground stacking and environmental pollution prevention and control. In order to study the diffusion mechanism of slurry in the collapse zone, a physical simulation experimental device was designed, the filling process was visualized, and the flow and diffusion law of the filling slurry in different regions was obtained by grouting and filling experiments and numerical simulations of broken gangue with different particle size ratios. The collapse zone model of the compaction zone and the under-compacted zone was established, and the flow and diffusion law of the slurry filling the collapse zone under different pressures and different porosities was obtained. The results show that the slurry presents a dynamic state of continuous superposition and mutual reinforcement in the three stages of vertical inrush, horizontal diffusion and axial diffusion. Under the same grouting pressure, the smaller the porosity of the gangue accumulation, the slower the flow speed of the slurry, and the lower the filling efficiency. The increase of grouting pressure can promote the flow and diffusion of slurry between pores and improve the filling effect. When the grouting pressure reaches 1.00 MPa, the filling effect is the best.