A new countermeasure method of jet grout columns with horizontal slab in liquefaction mitigation is introduced in this study. In order to investigate the effectiveness of the proposed mitigation method, numerical studies on unimproved and improved ground were separately performed. Additionally, the validations of the numerical model geometry with boundary conditions and parameters used in UBC3D liquefaction model were conducted prior to the analyses to confirm the reliability of the numerical results. The effectiveness of the soil improvement method was assessed by comparing the changes in excess pore water pressure ratio, acceleration as well as distribution of shear stress and shear strain in the liquefiable soil before and after ground improvement. The results show that the new liquefaction mitigation method offers optimistic effect on control of shear deformation and excess pore water pressure.