Abstract: The lap spot welding of 6061 aluminum alloy and DP590 galvanized steel was carried out by cold metal transfer welding (CMT) technique. The microstructure and phase composition of joint interface were studied,and the formation mechanism of intermetallic compounds was discussed. The results show that the weld of the joint was well formed. There was an intermetallic compound transition layer with a thickness of about 6 μm on the interface of welding joint, the transition layer interface near galvanized steel side was smooth, and there were a lot of columnar crystals on the interface near the melting zone side. The content of iron and aluminum elements in the transition layer was almost unchanged. The microstructure near galvanized steel side was α-Fe solid solution containing aluminum, Fe₂Al₅ phase was formed near melting zone side, and fine needle-like FeAl₃ phase was formed in melting zone.