Abstract: Ground high-pressure advanced regional grouting has become the preferred engineering method for the prevention and control of water disasters in coal mines. The diffusion mechanism of grout has received widespread attention, and the design of branch hole spacing mostly relies on the on-site experience, which has a certain degree of blindness and restricts the effectiveness and efficiency of the project. Based on the actual working conditions of grouting site in Hengyuan Coal Mine, Huaibei Coalfield, the numerical simulation was performed with the COMSOL Multiphysics software for the high-pressure grout diffusion in the floor area of the ground directional drilling coal seam, the influencing factors of grout diffusion were analyzed, and the mechanism of ground high-pressure grout diffusion was discussed. The results are as follows: (1) Fractures are developed in the third thin layer of limestone in the Carboniferous Taiyuan Formation (hereinafter referred to as “the third limestone layer”) of the coal seam floor. It is found through simulation that the fracturing effect is likely to occur during 8-12 MPa high-pressure grouting, during which the permeability coefficient increases by about 4 orders of magnitude. (2) Without considering the influence of gravity, the grout diffusion distance to the periphery is equal with the branch hole as the center, and the grout density is large near the branch hole, with the volume fraction more than 0.2. When the gravity is considered, the horizontal grout diffusion range is relatively increased, and the grout mainly diffuses downward vertically, which is “short up and long down”. (3) The grout diffusion simulation shows that the diffusion range is 41.2 m. Definitely, the field grout diffusion range is measured to be 38.3-44.0 m, which verifies the correctness of the grout diffusion model. (4) The range of grout diffusion is negatively correlated with the relative density, hydrostatic pressure, dynamic viscosity, yield strength and porosity of grout, and positively correlated with the width of fracture and permeability coefficient. Specifically, the smaller the relative density of grout is, the more obvious the response of its diffusion range is. With the increase of dynamic viscosity, the diffusion range decreases more and more slowly. The research results could provide a reference for the reasonable design of the grouting treatment project in the water hazard area of the coal measures floor, with good reference and application value.