Emissive　spectrum　diagnosis　is　important　to　study　welding　arc　phenomenon.　Limited　by　experimental　condition　in　the　past,　the　knowledge　of　spectrum　dynamic　diagnosis　is　not　enough　and　most　of　the　diagnosis　results　come　from　stable　TIG　arc.　In　this　paper,　based　on　modern　ICCD　spectrum　instrument,　a　new　type　of　arc　spectrum　dynamic　diagnosis　system　is　established　and　a　new　diagnosis　method　based　on　time-control　and　statistic-average　is　developed　to　observe　AC　dynamic　welding　arc　continuously.　The　half-height　width　and　wavelength　shifting　of　AC　TIG　arc　spectrum　line　can　be　observed,　furthermore　electron　density　and　Ar　atom　excitation　temperature　can　be　obtained.　This　dynamic　observation　is　quite　significant　to　study　the　time-characteristic　of　arc　(especially　alternative　current　arc).