Water-based　biocarbon　as　an　electrode　material　of　supercapacitor　for　energy　storage　equipment　has　been　widely　studied　because　of　its　simple　preparation,　low　cost,　environmental　friendliness　and　good　cycle　stability,　but　its　low　specific　capacitance　and　energy　density　are　the　keys　to　hinder　its　development.　In　this　study,　2D　biocarbon　nanosheets　from　kapok　fibers　were　prepared　via　KOH　activation.　ACKF-2,　an　as-prepared　activated　biocarbon　with　an　alkali-carbon　mass　ratio　of　5:1,　had　a　very　high　specific　surface　area　of　3359.2　m2/g　and　a　high　pore　volume　of　2.1415　m3/g.　Thus,　ACKF-2　exhibits　a　high　specific　capacitance　of　405　F/g　at　a　current　density　of　1　A/g　in　a　6　mol/L　KOH　electrolyte.　To　further　improve　the　electrochemical　performance　of　ACKF-2,　two　redox　additives,　K3Fe(CN)6　and　K4Fe(CN)6,　were　chosen　as　novel　redox　electrolytes.　At　a　current　density　of　10　A/g,　ACKF-2　showed　a　superhigh　specific　capacitance　of　3380　F/g　in　the　1　mol/L　H2SO4　electrolyte　with　adding　K3Fe(CN)6　and　K4Fe(CN)6.　A　symmetrical-supercapacitor　device　consisting　of　ACKF-2　and　the　new-type　redox　electrolyte　can　achieve　an　extremely　high　energy　density　of　264.1　Wh/kg　at　a　high　power　density　of　4593　W/kg.　Therefore,　this　study　provides　a　new　energy　storage　scheme　that　can　greatly　improve　the　water-based　carbon　supercapacitor.