During the construction process of bored pile, it is significant to prevent excessive infiltration and diffusion of slurry against the wall protection to maintain the stability of the pile hole. First, by considering the variation of permeability coefficient of unsaturated soil and the viscosity of slurry, the infiltration shape at the bottom of the pile hole was assumed to be bowl-shaped and a model for slurry infiltration and diffusion was established. Based on the principles of fluid mechanics in porous media, an analytical formula for the diffusion distance of the slurry was obtained. The slurry infiltration and diffusion model was then compared and validated. The results shows that the diffusion distance assumed to be spherical is 2.7 times that assumed to be bowl-shaped. As the pile hole radius varies from 0.1 m to 1.0 m, the infiltration range increases by 2.2 to 2.6 times. When the moisture content of medium-coarse sand is greater than 24% or the matrix suction is less than 6 kPa, the effect of unsaturation on the infiltration range is less than 10%. For slurry initial viscosity n_p0 ≤25 mPa·s, increasing the initial viscosity n_p0 can control the diffusion range of the slurry in the soil layers. Furthermore, suitable slurry parameters and types are proposed based on different types of strata. The findings of this study can provide a theoretical basis for controlling the infiltration range of slurry and have practical implications for engineering projects.