This　paper　presents　a　strengthening　method　for　reinforced　concrete　(RC)　columns,　in　which　an　additional　RC　column　is　attached　to　the　original　RC　column.　This　method　is　similar　to　the　reinforcement　method　by　enlarging　the　column　section　on　one　side.　To　investigate　the　mechanical　behavior　and　enhancement　performance　of　the　strengthened　columns　under　monotonous　horizontal　loads,　finite　element　analysis　was　conducted　through　the　commercial　software　ABAQUS.　In　the　analysis,　loading　directions,　section　size　of　strengthened　part,　and　pre-stressing　ratios　were　selected　as　parameters　to　reveal　the　mechanical　behavior　of　the　strengthened　columns.　Unreinforced　column　was　also　simulated　for　comparison.　The　analytical　results　show　that　the　bearing　capacity　in　two　principal　directions　of　the　strengthened　columns　increases　in　different　degrees,　the　deformation　ability　is　affected　slightly.　And　the　lager　the　section　size　of　strengthened　part　is,　the　more　increase　of　bearing　capacity　in　two　principal　directions　will　be　achieved,　the　deformation　ability　is　also　affected　slightly.　When　applying　pre-stress　to　the　strengthened　part,　the　increase　of　bearing　capacity　is　more　significant,　but　the　pre-stress　ratios　have　different　impacts　on　the　bearing　capacity　in　two　principle　directions,　the　deformation　ability　decreases　when　increasing　the　pre-stressing　ratio.