Abstract: The reasonable parameters setting of grouting and sealing boreholes is key to improve the efficiency of gas extraction. In response to the poor effect of grouting and sealing boreholes drilled by large-diameter drilling bit under multistage excavation, the fracture evolution and slurry diffusion laws of slurry are studied, both the fracture evolution laws, and the slurry diffusion laws based on the fractures distribution of multistage large-diameter drilling boreholes is obtained. The results show that when the final size of boreholes is 400 mm diameter, comparing with the one-time borehole-forming method, the radius of drilling fracture area of the multistage borehole-forming method increases from 0.50 m to 0.75 m, which shows the latter drilling method can provide more fissure channels for gas migration. The number of shear fractures generated in the coal body around the borehole is significantly higher than that of tensile fractures, proving that shear stress is the main reason for the formation of fractures. When the grouting pressure is increased from 2.00 MPa to 3.00 MPa, the diffusion distance is increased from 0.644 m to 0.657 m while the maximum slurry diffusion velocity is increased from 0.10 m/s to 0.18 m/s; when the consistency coefficient k is increased from 13 Pa·s to 21 Pa·s, the maximum slurry diffusion velocity is reduced from 0.14 m/s to 0.08 m/s while the diffusion distance drops from 0.662 m to 0.644 m. According to the results of numerical simulation and empirical formula verification, when applying high-pressure holding grouting technology to the coal mine working face in Shanxi Province, it is confirmed that the best grouting diffusion effect can be achieved when the optimal grouting pressure is 3.00 MPa and the optimal water/cement ratio is 1∶1.2, the radial diffusion radius of grouting can reach between 0.597 m and 0.615 m. The results can provide the theoretical basis for determining the grouting parameters for sealing multistage large-diameter drilling boreholes.