The current research of narrow-gap gas metal arc welding (NG-GMAW) primarily focuses on improving the sidewall fusion and avoiding the lack-of-fusion defect.However,the high cost and operation difficulty of the methods limit the industrial application.in this study,small amount of active gases CO2 and O2 were added into pure argon inert shield-ing gas to improve the weld formation of pulsed-current narrow-gap gas metal arc welding (NG-GMAW) of mild steel.Their effects on droplet transfer and arc behavior were investigated.A high-speed visual sensing system was utilized to observe the metal transfer process and arc morphology.When the proportion Of CO2,being added into the pure argon shielding gas,changes from 5％ to 25％,the metal transfer mode changes from pulsed spray streaming transfer to pulsed projected spray transfer,while it remains the pulsed spray streaming transfer when 2％ to 10％ O2 is added.Both CO2 and O2 are favorable to stabilizing arc and welding process.O2 is even more effective than CO2.However,O2 is more likely to cause slags on the weld surface,while CO2 can improve the weld appearance in some sense.The weld surface concavity in NG-GMAW is greatly influenced by the addition of active gas,but the weld width and weld penetration almost keep constant.This study proposes a new method which is beneficial to improving the weld bead formation and welding process stability.