Abstract: Welding thermal cycle curves of 6005A-T6 aluminum alloy by using tandem MIG welding and laser-tandem MIG hybrid welding was tested individually. The softening of the welding joints was studied through welding thermal simulation with heat treatment method. The test results demonstrated that the notable softening in HAZ began as long as the peak temperature of welding thermal cycle curve exceeded 260℃, and the most serious softening in HAZ occurred while the peak temperature of welding thermal cycle curve was up to 350℃. The unstable phase of β" and β', the main strengthening phase of 6005AT6 aluminum alloy, transformed to β(Mg₂Si) which aggregated and grew to block was the main reason why the softening happened. The hardness of the joints in HAZ, where the peak temperature of welding thermal cycle curve was between 260-500℃, could not recover totally through ageing treatment. The detention time beyond 260℃ on the thermal cycle curve of lasertandem MIG hybrid welding joints was much shorter than that of tandem MIG welding joints, so that the softening of laser-tandem MIG hybrid welding joints was little slighter than that of tandem MIG welding joints.