Abstract: Using scanning electron microscopy, energy spectrum, and real-time acquisition of temperature fields, the effects and mechanisms of Si content in the welding wire on the sensitivity to thermal cracking of AA6063 aluminum alloy GMAW welded joints were studied. The results show that when the welding wire is pure aluminum, slender welding cracks appear in the center of the weld of the fishbone sample; when the Si content in the welding wire is 4.5% ~ 6%, the length of the crack is short, but the cracking distance is obviously increased; when the Si content in the welding wire reached 11% ~ 13%, no cracks appeared in the sample weld. With the continuous increase of Si content, the solidification temperature range of alloy prone to crack increases first and then decreases. The difference of Si content in welding wire will also affect the fluidity of molten metal at the later solidification stage, making the phase composition and morphology at the grain boundary of weld obviously different. At the same time, the increase of Si content will make the cooling rate of the joint increase first and then decrease, thus causing the stress state to change, and the thermal crack sensitivity increases first and then decreases.