Ground improvement using jet grout columns is a well-known technique to mitigate liquefaction hazard in sand stratum under existing building. However, the performance of conventional jet grout reinforcement technique has not achieved the sufficient level yet in terms of reducing shear strains and excess pore water pressure generated within the liquefiable soil layer. Therefore, a new countermeasure method, using small diameter jet grout column with additional horizontal slab, is introduced to control the shear deformation and excess pore pressure more effectively. To determine the efficiency of the new countermeasure method, numerical studies on unimproved and improved ground were separately performed in this study. The effectiveness of jet grout column with horizontal slab was evaluated by comparing the changes in excess pore water pressure, acceleration as well as distribution of shear stress and shear strain in the liquefiable soil before and after improvement. The results showed that the new liquefaction mitigation method offers positive effect on control of excess pore water pressure and shear deformation.