Abstract: Variable polarity TIG welding of aluminum alloy was investigated with sine and square wave power supply output, respectively. Arc shape was achieved by high-speed camera. Welding current and arc voltage were acquired by Hannover welding quality analyzer. The experimental results indicate that the arc is less stable with sine waveform output than that with square wave under lower welding parameters based on distribution of voltage probability density and high speed photography. With the increasing time ratio of electrode positive period with sine waveform output, the arc cleaning area became wider, while the weld bead width kept constant since the arc was less concentrated. The weld bead width with both sine and square waveform outputs increased under large welding parameters. The results show that the arc energy was greater in electrode positive period than in electrode negative period.