Abstract: Two cores melting speed can keep consistence is very important for twin electrode single arc welding to be stabilized.The melting consistency of two cores is influenced by electrode type,welding current etc.The sufficient condition of two cores melting coincidence is the effective anode drop equal to the effective cathode drop,the more the difference of two values the more the melting inconsistent degree.The arc has a self-regulating effect on melting of two cores,that is,during the arc burning,even if the ends of two cores have different length in the arc,two cores can keep uniformly melting speed,and the arc can stably burn.The principle of the self-regulating effect is that different heat production of cathode and anode make two cores length in the arc different.The longer weld core stretching into the arc column gets more heat from the arc column to accelerate its melting.In general,the self-regulation effect of arc increases with the reducing of two-core span and increasing of welding current.