Potassium-ion　batteries　have　attracted　increasing　attention　for　next-generation　energy　storage　systems　due　to　their　high　energy　density　and　abundance　of　potassium.　However,　the　lack　of　suitable　anode　highly　hampers　its　practical　application　due　to　the　large　ionic　radius　of　K+.　Herein,　a　Se3P4@mesoporous　carbon　(Se3P4@C)　composite　is　reported　as　a　high-performance　anode　for　potassium-ion　batteries.　The　Se3P4@C　composite　is　synthesized　through　an　in　situ　combination　reaction　between　red　phosphorus　and　Se　within　a　porous　carbon　matrix.　In　this　way,　the　nano-sized　Se3P4　is　well　confined　in　the　porous　carbon　and　thus　exhibits　a　close　contact　with　the　carbon　matrix.　This　can　significantly　improve　the　conductivity　and　alleviate　the　volume　change　during　the　cycling　process.　As　a　result,　the　Se3P4@C　exhibits　a　high　reversible　initial　capacity　of　1036.8　mAh　g(-1)　at　a　current　density　of　50　mA　g(-1)　as　well　as　an　excellent　cycle　performance　with　a　capacity　decay　of　0.07%　per　cycle　over　300　cycles　under　1000　mA　g(-1).　In　terms　of　high　specific　capacity　and　stable　cycling　performance,　the　Se3P4@C　anode　is　a　promising　candidate　for　advanced　potassium-ion　batteries.