Abstract: The flowing of groundwater has a significant impact on grout diffusion in the process of permeation grouting. Based on this phenomenon, the process of permeation grouting under hydrodynamic condition was described by two-phase seepage theory, and a numerical simulation was conducted by establishing a corresponding finite-element model to study the law of grout diffusion and the factors affecting the grout diffusion range. Results show that grout diffusion form is shaped in a“U”along the flow direction under the hydrodynamic condition, and the dynamic water field restricts the range of slurry diffusion; there are significant differences between lateral grout diffusion and longitudinal grout diffusion, and between grout diffusion with the flow direction and against the flow direction; grouting pressure and grout velocity decrease sharply near the grouting hole; the underground water flow field and grouting pressure are the main controlling factors affecting the range of grout diffusion; and the range of grout diffusion has a positive correlation with grouting pressure and a negative correlation with the velocity of the flowing water. Suggestions are proposed to improve the design methods and technologies of permeation grouting under hydrodynamic conditioin.