Abstract: This paper describes the state and behaviours of a plasma arc re-constricted by a dual-cusp magnetic field.The load characteristics,the current density,the pressure distribution and the are stability indexes were measured.On the basis of investigating the physical features of the re-constricted plasma arc,the melt-in mode,and the key holemode plasma arc welding on stainless steel plate had been tested.It is shown that when re-constricted,the energy density of the plasma arc is greatly increased,the current distribution and pressure distribution,thus the weld configuration,can be easily adjusted,the "dual-arc" occurring current and the limited welding thickness for keyhole mode plasma arc welding increase.All these characteristics make a plasma arc more flexible and give many advantages to its application.