Abstract: The microstructure and bonding properties of the welded marine ultrathin aluminum-steel composite transition joints are studied experimentally. The results show that when the interface temperature is below 300℃, the bonding properties of the aluminum-steel interfaces of the welded transition joints are similar to those of the original state. The structure of welded aluminum-steel interface is normal, and the metal-to-metal compounds of Fe_xAl_y at the interface are not thickened.