A　sheath　is　the　transition　region　from　plasma　to　a　solid　surface,　which　also　plays　a　critical　role　in　determining　the　behaviors　of　many　lab　and　industrial　plasmas.　However,　the　cathode　sheath　properties　in　arc　discharges　are　not　well　understood　yet　due　to　its　multi-scale　and　kinetic　features.　In　this　letter,　we　have　adopted　an　implicit　particle-in-cell　Monte　Carlo　collision　(PIC-MCC)　method　to　study　the　cathode　sheath　in　an　atmospheric　arc　discharge　plasma.　The　cathode　sheath　thickness,　number　densities　and　averaged　energies　of　electrons　and　ions,　the　electric　field　distribution,　as　well　as　the　spatially　averaged　electron　energy　probability　function　(EEPF),　are　predicted　self-consistently　by　using　this　newly　developed　kinetic　model.　It　is　also　shown　that　the　thermionic　emission　at　the　hot　cathode　surface　is　the　dominant　electron　emission　process　to　sustain　the　arc　discharges,　while　the　effects　from　secondary　and　field　electron　emissions　are　negligible.　The　present　results　verify　the　previous　conjectures　and　experimental　observations.