Antimony　(Sb)-based　anode　materials　have　recently　aroused　great　attention　in　potassium-ion　batteries　(KIBs),　because　of　their　high　theoretical　capacities　and　suitable　potassium　inserting　potentials.　Nevertheless,　because　of　large　volumetric　expansion　and　severe　pulverization　during　potassiation/depotassiation,　the　performance　of　Sb-based　anode　materials　is　poor　in　KIBs.　Herein,　a　composite　nanosheet　with　bismuth-antimony　alloy　nanoparticles　embedded　in　a　porous　carbon　matrix　(BiSb@C)　is　fabricated　by　a　facile　freeze-drying　and　pyrolysis　method.　The　introduction　of　carbon　and　bismuth　effectively　suppress　the　stress/strain　originated　from　the　volume　change　during　charge/discharge　process.　Excellent　electrochemical　performance　is　achieved　as　a　KIB　anode,　which　delivers　a　high　reversible　capacity　of　320　mA　h　g-1　after　600　cycles　at　500　mA　g-1.　In　addition,　full　KIBs　by　coupling　with　Prussian　Blue　cathode　deliver　a　high　capacity　of　396　mA　h　g-1　and　maintain　360　mA　h　g-1　after　70　cycles.　Importantly,　the　operando　X-ray　diffraction　investigation　reveals　a　reversible　potassiation/depotassiation　reaction　mechanism　of　(Bi,Sb)　↔　K(Bi,Sb)　↔　K3(Bi,Sb)　for　the　BiSb@C　composite.　Our　findings　not　only　propose　a　reasonable　design　of　high-performance　alloy-based　anodes　in　KIBs　but　also　promote　the　practical　use　of　KIBs　in　large-scale　energy　storage.　©　2019　American　Chemical　Society.