Abstract: A new welding process,the magnet constricted vacuum plasma arc (MCVP arc) welding,is introduced in this article.The arc is obtained by applying a magnetic field coaxial with the electrode to constrict an open arc in medium vacuum.It is observed in the experiments that along with the lowering of the atmosphere pressure,the open arc gets larger and larger,and the current density in the arc root zone becomes lower and lower until an instable "side arc" appears and then the arc goes out.The open arc in medium vacuum can be hardly constricted by means of the mechanical constricting method for common plasma arc.However,the vacuum arc can be constricted effectively by a coaxial magnetic field and the constricting effect gets more obvious as the atmosphere pressure decreases.When the pressure is lower than 1.33×10² Pa,a megnetic field of one over several tens of Tesla can make the diameter of the MCVP arc column from top to bottom nearly the same and only slightly thicker than the electrode.As a heat source,the MCVP arc can weld the common and active metals.Its penetrating ability is higher than that of common TIG arc.The energy distribution can be adjusted by regulating the magnetic field density.There are distinctive advantages of this welding process over the conventional TIG,plasma arc and electron beam welding.