Abstract: A new type of square wave A.C.arc welding source is introduced.Its main circuit consists of an ordinal D.C. arc welding source and a thyristor inverter converting D.C. into square wave A.C. When the alternating welding current passes zero,arc can be easily reignited at 10 A,even though the open circuit voltage of the D.C. source is as low as 70 V.The welding parameters such as the number of frequency,time period and amplitute of the alternating welding current can be selected within an reasonable range at will.Technological experiments have confirmed that this new type of welding source has many advantages,such as the width of arc cleaning and the heat input to the workpiece can be controlled and the tungsten electrode loss is very little during the TIG welding of aluminium alloys,thus resulting in more effective use of arc heat and force to meet some special requirements.Furthermore,this new welding source can be used in welding with base electrodes and decrease arc blow and give stable arc process.