This　paper　investigated　the　arc　characteristics　in　pulsed　plasma　gas　of　plasma　arc　welding　using　a　synchronous　image-　and　electrical　signal-acquisition　system.　The　arc　sizes　(including　the　arc　length　L,　the　arc　width　on　the　water-cooling　copper　plate　surface　W-a,　and　the　arc　width　under　the　nozzle　W-c)　and　arc　voltage　changed　regularly　with　the　plasma　gas　periodically　on/off.　And　the　changing　trends　of　arc　sizes　were　affected　by　the　plasma　gas　flow　rate　and　the　plasma　gas　switching　frequency.　The　changing　scale　of　arc　voltage　was　proportional　to　the　plasma　gas　flow　rate　and　was　inversely　proportional　to　the　plasma　gas　switching　frequency.　The　average　plasma　arc　welding　(PAW)　and　plasma　arc　welding　with　pulsed　plasma　gas　(PPG-PAW)　arc　voltages　were　equal　when　the　plasma　gas　flow　rate　was　the　same.　The　arc　pressure　also　presented　cyclical　changes　during　the　plasma　gas　on/off,　and　the　changing　scale　and　the　axial　mean　value　were　inversely　proportional　to　the　plasma　gas　switching　frequency.　The　periodically　oscillating　arc　and　arc　pressure　would　be　beneficial　to　the　refinement　of　grain　structure　during　the　welding　process.