<p>Gangue grouting and backfilling for the mining space is an effective measure to address the substantial discharge of gangue in coal mining and the extensive mining space. The flow and diffusion characteristics and laws of gangue slurry directly determine the success or failure of gangue grouting and backfilling mining. In this study, a self-designed gangue grouting and backfilling simulation test system was developed with a visual backfilling function, the flow and diffusion characteristics and distribution shape of gangue slurry in the mining space were obtained, and the influence of porosity, injection pressure, and slurry properties on the diffusion range of gangue slurry was revealed. The results show that the gangue slurry undergoes both horizontal and vertical diffusion in the mining space and the diffusion process is divided into three stages: rapid diffusion stage, stable seepage stage, and horizontal expansion stage. The distribution shape of the gangue slurry is a combination of circular cones of different sizes. The linear growth relation between the diffusion range and vertical depth of gangue slurry gradually transforms into a nonlinear growth relation with the increase of grouting pressure. The increase of porosity enhances the vertical diffusion of gangue slurry and the increase of injection pressure prolongs the amplitude of horizontal diffusion. Besides, the presence of coarse-grained gangue has a significant influence on the slurry diffusion range. When the outlet pressure of the actual grouting is between 2 and 6&#xa0;MPa, the diffusion range of the gangue slurry is about 300&#xa0;m. This research provides a theoretical foundation for investigating the control effect of surface deformation and the design of key parameters in gangue grouting and backfilling within mining spaces.</p>