Abstract: A variable polarity plasma arc (VPPA) welding system that has single power supply and double pulse hybrid modulation is developed. High-low frequency pulse is modulated on the basis of typical VPPA welding power, in which high frequency is from 1 kHz to 5 kHz, low frequency is from 1 Hz to 2 Hz and double pulse consisting of both high and low frequency can be generated. With the utilization of tensile test, SEM observation, the welding quality and mechanical properties of the welded 3003 aluminum joints are analyzed. The test results show that because of the effect of stirring and impacting on molten pool exerted by the periodicity variation of pulse current, the tensile strength of the weld joint is improved, the weld grains are refined and the fracture dimples are uniform and dense. Thus, the quality of the aluminum alloy welds is improved.