Abstract: To analyze the horizontal penetration grouting process in weakly cemented sand gravel layer from the mesoscopic perspective, a numerical calculation model of horizontal grouting was established by using particle flow software. The influences of slurry viscosity, grouting pressure and grouting time on slurry diffusion radius and sample porosity were studied based on orthogonal and single factor test. The relationship between diffusion radius and the three factors can be obtained by fitting the expressions with regression analysis method. The simulation results show: The slurry diffusion radius is positively correlated with grouting time and grouting pressure, and negatively correlated with slurry viscosity, the most significant factor affecting the slurry diffusion radius is the slurry viscosity, followed by the grouting pressure and grouting time; The slurry diffusion rate decreases with time, under the condition of grouting pressure of 1.0 MPa, the slurry diffusion rate at 0.15 h is 3.44 m/h, which decreases to 0.32 m/h at 3 h; The sample porosity increases with the increase of grouting time and grouting pressure, and decreases with the increase of slurry viscosity; Compared with single hole sequential grouting, the slurry diffusion range of flower-tube grouting is larger at the same time, and the slurry diffusion is a coherent whole. By comparing the engineering examples, it is found that the general diffusion law of permeated grouting in sand gravel layer can be accurately reflected by the numerical simulation results of particle flow.