We　report　that　complete　spin　polarization　and　controllable　spin　polarization　of　carriers　can　be　simultaneously　realized　in　the　Heusler　alloy　Mn2CoAl　simply　by　applying　external　pressures　based　on　first-principles　studies.　At　ambient　conditions,　Mn2CoAl　is　a　ferromagnetic　spin-gapless　semiconductor　(SGS)　with　complete　spin　polarization.　Under　hydrostatic　pressures　up　to　40　GPa,　Mn2CoAl　undergoes　a　series　of　electronic　transitions　from　SGS　with　spin-up　as　a　conducting　channel　to　a　ferromagnetic　semiconductor　and　then　to　SGS　with　spin-down　as　a　conducting　channel　and　finally　to　a　half　metal,　during　which　the　magnetic　moment　remains　as　2　mB.　Such　rich　electronic　transitions　are　attributed　to　different　responses　of　the　spin-up　and　spin-down　electrons　under　pressure.　This　work　highlights　a　desirable　way　to　control　the　carrier＇s　spin　polarization　and　provides　a　new　insight　into　the　electron　behavior　in　Mn2CoAl　related　Heusler　alloys　under　pressure.