Abstract: Based on the novel hybrid ultra-high frequency pulse square-wave variable polarity gas tungsten arc welding (HPVP-GTAW) technique,the effect of ultra-high frequency pulse square-wave current on the microstructure and mechanical properties of 5A06 aluminum alloy joints was investigated. The experimental results show that the frequency of pulse square-wave current had an important influence on the microstructure characteristics of the resultant weld. With the pulse current frequency between 40 kHz and 60 kHz,the amount of β(Mg₂Al₃) grains increased and the softening of joints decreased,the mechanical properties of joints were improved obviously. The tensile strength and percentage elongation of the joint reached more than 93% and 80% of the base metal,respectively. While the pulse frequency was kept constant at 20 kHz,the variation of pulse current amplitude and duty cycle had not obvious influence on the weld quality.