The　seismic　performance　of　irregular　pentagonal　concrete-filled　steel　tube　(CFT)　megacolumns　is　very　different　from　that　of　circular　or　square　CFT　columns.　Six　irregular　pentagonal　CFT　megacolumn　specimens,　with　multiple　cavities　each,　derived　from　a　tall　building　are　tested　under　constant　axial　compression　and　low　lateral　cyclic　loading.　The　main　studied　parameters　are　the　lateral　loading　directions　and　bottom　cross-section　constructions.　The　types　of　constructions　investigated　are　normal,　corner-steel-angle　strengthened　and　simplified　discontinuous　partial　steel　plate　fabrications.　The　influence　of　the　parameters　on　the　failure　mode,　bearing　capacity,　elastic-plastic　deformability,　and　energy　dissipation　is　analysed.　The　failure　features　are　fractures　of　the　bottom　steel　plates　caused　by　the　bending　force.　Both　the　normal　and　the　strengthened　CFT　columns　exhibit　better　seismic　performance　than　the　simplified　columns　and　have　a　8.9%-34.7%　greater　bearing　capacity　and　a　24.5%-42.2%　stronger　elastic　deformability.　Therefore,　utilisation　of　the　simplified　CFT　columns　should　be　limited　due　to　its　weakened　ductility　and　energy　dissipation　ability　when　using　a　steel　ratio　similar　to　those　used　for　the　other　column　types.　Finally,　a　simplified　strength　model　for　an　irregular　pentagonal　CFT　megacolumn　is　proposed,　and　the　resultant　prediction　is　conservative　but　matches　the　test　result　well.