Abstract: For the black oxide slag defects in the process of traditional single-supplied VPPA-GTAW double-sided keyhole welding of aluminum alloy, the defect mechanism was studied by using electrical signal sensing, image acquisition and image processing, and the current-voltage characteristics and arc shapes before and after the keyhole formation were obtained. The results show that the cross-sectional area of the penetrating arc after the keyhole formation decreases significantly, and the cathode spots on the workpiece disappear dramatically, which results in drop of reversed polarity voltage of workpiece and weak of cathode atomization effect. In order to eliminate the black slag defects, a new single-supplied VPPA-GTA dual-status arc welding method was proposed. By optimizing the output current direction of DCEN section and DCEP section of the variable polarity plasma arc power supply, the positive polarity plasma arc can be formed between plasma welding gun and workpiece, and the reverse polarity penetration arc can be formed between plasma welding gun and TIG welding gun. The dual-status arc owns the advantages of both the oxidation film cleaning effect of VPPA and the energy concentration of single power supply perforated arc, and good weld formation can be obtained.