Rock grouting is a common measure to reduce the seepage through conductive fractures in the rock mass around tunnels. Two types of grouting are normally carried, pre-excavation grouting and postexcavation grouting. Pre-grouting, commonly applied in Scandinavian tunnels, is used to seal the conductive fractures around the tunnel before the excavation of tunnel sections. In post-excavation grouting, which is dedicated to seal the remaining leakage in the excavated tunnel sections, the injected grout often encounters large seepage in rock fractures. Previous experiments have shown that the grout can be washed out easily when the grout is fresh even though the injected grout has initially sealed the fracture. One of the most significant phenomena for the water to “break up” the grout is viscous fingering. Viscous fingering occurs when certain conditions enable interface instability between the water and the cement-based grout. In this paper, the authors aim to evaluate if viscous fingering can be avoided under pre- and post-grouting conditions. For this purpose, computational fluid dynamics (CFD) simulations using the software Ansys Fluent is carried out. The simulation results demonstrating viscous fingering between water and cement-based grout are analyzed and discussed. Based on the results, suggestions on the grouting strategy with respect to pre- and post-grouting are provided to deal with the potential issues related to viscous fingering.