Abstract: The advantage of variable polarity plasma arc welding is that the characteristic parameters such as the frequency, current amplitude and DCEN/DCEP ratio can be adjusted at will to meet the need of cathodic cleaning and keyholing. The status of realization of variable polarity current waveform, arc stability, cathodic cleaning mechanism and keyhole detection is introduced in this paper. To ensure the synchronization between primary and secondary circuit is the key of dual-inverter. Dual-passage can eliminate the interference between main arc and pilot arc, and decrease the deterioration of tungsten electrode. Ion sputtering or ion bombardment, dielectric breakdown and momentary melting and evaporating are the main theories of cathodic cleaning mechanism. It is more important to master the regularity of cleaning. Of all recent developed keyhole detection methods, front side detection and multi-sensing information is still the leading direction.