Abstract: The theory of stability of a liquid with free surface was applied to understand the instability phenomenon in bead formation.The experiment and analysis were carried out to discuss the effects on the bead formation for different current phases.The result showed that with the pulse current of the host and the accessory changed synchronously,the twin arc extinguished periodically.Then the heat which was transferred into the molten pool was decreased.This helped to avoid the molten pool to be too long and avoided the unstability of the liquid metal.When the pulse current of the host and the accessory changed alternately,the workpiece was heated by the arc throughout the welding process.As a result,the molten pool was very long,which gave way to the unstability of bead formation and pinch effect of the liquid metal,so that it finally leaded to the humping phenomenon.When the pulse current phase of the host and the accessory changed indefinitely,the arc burned and extinguished in random.At the same time,the twin arc extinguished and the molten pool solidified in time.But when the twin arc extinguished,the molten pool could solidify in time and an uneven weld appearance was formed.