To increase the service life of existing asphalt pavement, we considered the new method by compaction grouting as the construction method which the existing subgrade can be improved without replacing the pavement structure. We investigated the various influence factors on subgrade improvement effect and reinforcement effect on bearing capacity of existing asphalt pavement by FEM analysis. The following result were obtained. Placement style of the improvement column and its depth in subgarade largely affect on subgrade improvement and the service life of existing asphalt pavement can be increased longer than design life in new constructin by compaction gruoting.