Welding　arc,　as　a　heat　source,　converts　electric　energy　to　heat　energy　by　gas　discharging,　and　the　scale　of　the　arc　ionization　region　as　well　as　the　effective　range　of　the　arc　determines　the　distribution　of　its　energy　density.　In　this　paper,　the　quantitative　detection　of　argon　atoms　and　argon-ion　line　intensity　were　achieved　by　emission　spectroscopy　diagnostic　method,　and　the　arc　ionization　region　and　the　gas　atom　excitation　radiation　boundary　scale　were　obtained.　The　measurement　not　only　overcame　the　problem　that　the　arc　could　not　be　quantitatively　tested　with　camera　due　to　exposure　factors,　but　also　could　be　used　for　observation　of　pulsed　arc.　The　results　provide　the　basis　for　better　understanding　arc　physics　and　modeling　the　arc.