Abstract: The critical condition of dynamic force equilibrium of instantaneous short circuit (ISC) in GMAW (gas metal arc welding) based on droplet pressure, electromagnetic force, and surface tension was presented while the dynamic force was analyzed.Based on micro focus high speed photography technique and digital image processing technology, the data of the droplet radius were obtained. And the electric signal was combimed and analyzed.The results showed the short circuiting process reflects by the calculated data completely agreed with that from the images.The critical condition is proved.Further, the influence of the droplet shape on the ISC was analyzed based on this critical condition.If the radial radius was less than axial radius when the droplet touched with the wolten pool, the radial surface tension would be positive.The ISC startes and can not be reversed, which means the spindly droplet leads to ISC frequently.Lastly, the ISC phenomenawas classified and studied quantificationally.And the new explanation of ISC was proposed.