Abstract: In the process of coal mining, due to the influence of complex hydrogeological conditions, the engineering construction often encounters some geological disasters such as water inrush and collapse, which seriously restricts the safe construction and development of underground mines. As an effective means of underground water inrush disaster management, the grouting technology has been widely used in the field of coal mine water disaster prevention and control.To study the diffusion law and plugging mechanism of inclined fracture grouting under dynamic water condition, a dynamic water grouting test for inclined fracture was carried out by the self-developed dynamic water grouting test system. The Navier-Stokes equations and the governing equations of slurry-water interface were solved by finite element method, the grout diffusion shape and diffusion area at different moments during the grouting process were obtained, the distribution law of velocity field and pressure field in inclined fracture space was analyzed, and the results were compared with the experimental results. Based on the model test and numerical simulation results, the migration law of slurry in the key area of water plugging was revealed, and the key area diffusion model of water plugging by dynamic water grouting in inclined fracture was established. The results show that when the grouting pressure is constant, with the increase of fracture dip angle and dynamic water flow, the diffusion distance of slurry along the downwater direction increases rapidly to the outflow boundary. The diffusion distance of the slurry along the direction against the water and perpendicular to the flow direction decreases obviously.As a result, the diffusion area of the slurry decreases continuously, and the diffusion form of the slurry changes from round to oval, and finally reaches the outflow boundary in a U shape. In the process of grouting diffusion, with the increase of the dip angle of the fracture, the slurry continuously displaces the water through section and gradually forms the high velocity water through channel, the velocity distribution of fracture cross section shows the shape of steep ridge and wide valley. At the same time, the pressure curve changes synchronously with time at different height-difference positions of the inclined fracture, the pressure distribution along the forward and reverse water directions (line 1) shows a decreasing trend from the inlet to the outlet boundary of the moving water.The larger the dip angle of the crack is, the faster the pressure field decays.The pressure distribution is symmetrical along the direction perpendicular to the water flow (line 3), which shows that the pressure value is the largest at the grouting hole, and the pressure decay rate is the fastest around the grouting hole, with the increase of the distance from the grouting hole, the pressure decay rate gradually decreases and tends to be smooth. According to the zonal-layered diffusion mode of grout in fracture, the location and scope of the key area of water plugging are determined, and the error between the test results and the theoretical calculation value of the key area of water plugging is less than 10%, it shows that the diffusion model established in this paper can describe the diffusion process of slurry in the key area of water plugging. According to the relative position of the grouting hole and the key area of water plugging, a reasonable arrangement of grouting holes can effectively improve the effect of grouting water plugging. Therefore, some suggestions to improve the traditional grouting process are put forward.