Half-metallic　ferromagnet　has　attracted　intense　interest　due　to　its　unique　electronic　structure　and　potential　applications　in　spintronic　devices.　The　structural　properties,　electronic　structure,　and　magnetism　are　systematically　investigated　for　Ruddlesden-Popper　layered　Perovskite　Bi8Ba4Mn8O28　by　using　the　first-principles　calculations.　It　is　found　that　Bi8Ba4Mn8O28　is　a　half-metallic　ferromagnet　with　a　saturated　magnetic　moment　of　31　mu　B　f.　u.　1　at　its　ground　state　with　the　orthorhombic　Pbnm　symmetry,　which　ground　state　is　determined　by　the　symmetry　analysis,　and　the　calculations　of　the　total　energy,　the　formation　energy,　and　the　magnetic　exchange　interaction.　We　demonstrate　that　the　half-metallicity　of　Bi8Ba4Mn8O28　is　fairly　robust　against　large　uniform　and　tetragonal　distortions.　Furthermore,　the　surface　effect　on　half-metallic　properties　is　also　discussed.　It　is　shown　that　all　terminations　but　Mn1O　can　maintain　their　half-metallic　properties　and　present　good　robustness　of　halfmetallicity　with　rather　wide　half-metallic　gap.