Abstract: The reason that the reverse polarity time increased with the total current increase of SYNCROWAVE-300(s) power source in A.C. plasma arc welding of aluminium alloys was investigated in this paper.The result showed that the increase of the total current for this type of SCR-reactor welding power source with constant current output should be realized by increasing the reverse polarity output time of the power source,which will cause serious deterioration of tungsten in TIG or plasma arc welding of aluminium alloys with high welding current and make the arc and the weld formation unstable apparently.The conclusion of this phenomenon for SYNCROWAVE-300(s)welding power source not only posseses important referencial value for better understanding the power source,but also posseses more important guidce value for reforming this type of power source to be applied to keyhole plasma are welding of aluminium alloys.