Abstract: After coal seam mining, a large number of randomly distributed voids and cavities exist inside the caving zone formed by the collapse of overlying strata, resulting in a highly inhomogeneous porous medium structure in the goaf. To explore the migration and filling mechanism of slurry in such heterogeneous porous media, a systematic study was conducted on the grouting diffusion law of goafs. Based on the fractal theory, a seepage grouting diffusion model for porous media was established. A large-scale similar simulation test system for grouting was built. Three groups of broken gangue samples with different particle size distributions were prepared by using the Talbot continuous gradation equation with the index n of 0.3, 0.6 and 0.9, and quantitatively characterized by the uniformity coefficient C_u of 392.50, 19.81 and 7.32 respectively. In the tests, P·O 32.5 Portland cement slurry with a water-cement ratio of 0.6 was adopted, and filling tests were carried out under grouting pressures of 0.4 MPa, 0.6 MPa and 0.8 MPa. The results show that when the uniformity coefficient decreases from 392.50 to 7.32, both the maximum and minimum horizontal diffusion distances of the slurry decline, while the difference between them increases. It indicates that the improvement of medium uniformity fails to enhance slurry diffusion uniformity, but intensifies local differences. As the grouting pressure rises from 0.4 MPa to 0.8 MPa, the maximum horizontal diffusion distance increases whereas the minimum one decreases, which aggravates the inhomogeneity of the diffusion range. Under the combined action of gradation parameters and grouting pressure, obvious preferential seepage paths form during slurry filling. High grouting pressure further magnifies the control effect of medium uniformity on slurry diffusion and reduces the overall diffusion uniformity. The variation trend of slurry diffusion radius is consistent with the prediction of the theoretical model, which verifies the rationality of the model. Nevertheless, differences in pore connectivity and the existence of preferential seepage paths in actual media will affect the prediction accuracy of the theoretical model to a certain extent. This study reveals the influence laws of particle gradation and grouting pressure on slurry diffusion uniformity. It is pointed out that the matching relationship between medium inhomogeneity and grouting pressure should be fully considered in engineering practice to avoid grouting blind areas and excessive slurry concentration, so as to improve the scientificity and controllability of goaf grouting treatment projects.