In the case of grid-wall soil improvement methods, which are used as liquefaction countermeasure methods, it is necessary to analyze the seismic response to determine the extent of residual displacement that occurs after an earthquake. However, as the modeling and analysis of grid-wall soil improvements with 3D geometry using a 3D model takes time, it can be to use a quasi-3D analysis method that combines two sets of two-dimensional cross-sections under boundary conditions. Furthermore, in some cases, to make the quasi-3D analysis results equivalent to the 3D analysis results in large-scale projects, the stiffness of the grid walls used in the quasi-3D analysis method was reduced from the value used in the 3D analysis. Although, in this case, it is possible to make a determination regarding the degree of reduction in the wall stiffness by checking the consistency against the experimental results, it would be difficult to make a determination in regard to the degree of reduction in the wall stiffness through a consistency check with experimental results in every project. Thus, we prepared a chart that can easily determine the degree of stiffness reduction of a grid wall for the quasi-3D analysis method considering the grid spacing, liquefaction layer thickness, and ground conditions. Based on this, it was demonstrated that quasi-3D analysis methods can be easily used in grid wall soil improvement design.