Aiming at the problem that the existing theory of grouting can't accurately describe the real diffusion law of grout liquid in the network of gap and fissure way. From the view of continuity equation, this paper established a numerical model by use of PFC2D, and studied the influence of grouting pressure,grouting time,permeability on grout diffusion radius in sandy pebble layer. It summarized the law and used non-linear fitting to obtain the relational expression between these factors and grout diffusion radius.Combining injection water test with dig result,it got the resulr which is consistent which that of simulation.The result can provide a theoretical support and conduction for the construction of engineering.