In view of the fact that reinforcement through fissure grouting serves as a primary approach in geotechnical engineering, a three-dimensional rough single fracture grouting model and a three-dimensional random fracture network grouting model have thus been established by using COMSOL software, with a viscosity time-varying function introduced to simulate the grouting process from a three-dimensional single fracture to multiple fractures, and with the time variability of the grout taken into consideration. The results indicate that while the grouting pressure and grouting time are positively correlated with the diffusion range of the slurry, the viscosity of the slurry is negatively correlated. Moreover, the inclination angle of the fissure will change the diffusion pattern of the slurry; with the inclination angle increasing, the diffusion shape changes from circular to elliptical and eventually to droplet shape. The diffusion range obtained without taking into consideration the time-varying properties of the slurry viscosity is always greater than that obtained with the time variability of the slurry taken into consideration. Additionally, as the grouting time increases, the impact of the time-varying characteristics of the slurry viscosity on the diffusion distance becomes more significant.