The　objective　of　this　paper　is　to　determine　the　high-current　vacuum　arc　mode　transition　diagram　of　a　horseshoe-type　axial　magnetic　field　(AMF)　contact　with　long　contact　gap.　Drawn　arc　experiments　were　carried　out　in　an　L-C　discharging　circuit.　Horseshoe-type　contacts　with　contact　material　of　CuCr50　and　contact　diameter　of　120　mm　were　arranged　in　a　demountable　vacuum　chamber.　Full　contact　gap　was　60　mm.　The　experiment　current　level　ranged　from　4　to　40　kA,　stepped　by　4　kA.　Prospective　arcing　time　was　set　as　2,　5,　and　9　ms,　respectively.　The　evolution　of　vacuum　arc　behavior　was　observed　using　a　high-speed　camera.　Nonsynchronous　evolution　phenomenon　of　twin　parallel　arc　columns　was　observed.　With　current　level　ranged　from　24　to　40　kA,　the　maximum　nonsynchronous　duration　transiting　from　small-gap　constricted　column　mode　to　diffuse　column　mode,　from　diffuse　column　mode　to　diffuse　arc　mode,　and　from　diffuse　arc　mode　to　multicathode-spot　arc　mode　were　0.98,　1.09,　and　1.55　ms,　respectively.　The　nonsynchronous　duration　in　various　vacuum　arc　modes　was　determined　as　well.　Summarizing　all　arc　mode　transition　points　in　the　experiment,　a　vacuum　arc　mode　transition　diagram　of　the　horseshoe-type　AMF　contact　was　proposed.　Boundary　of　uncontrollable　arc　region　was　determined,　which　could　provide　criteria　of　maximum　arcing　contact　gap　design.　Besides,　an　effective　arcing　region　was　defined　according　to　distribution　of　AMF　and　arc　behaviors,　in　which　AMF　on　the　contact　surface　plane　was　higher　than　75%　Baxial-max.