In order to optimize the MIG welding procedure of aluminum alloy with longitudinal magnetic field and improve the mechanical property, effect of longitudinal magnetic field frequency on microstructure and properties of surfacing layer were investigated. The results show that when the additional longitudinal magnetic field was introduced into welding process, grain was refined, the wear resistance and tensile strength of the surfacing layer were improved as longitudinal magnetic field frequency increased. When frequency of magnetic field is 20 Hz and exciting current is 15 A, effect of grain refining is the best and the mechanical property is the highest. But when exciting current continue to increase, grain became coarse and the mechanical property decreased. So it is concluded that grain was refined and the mechanical property of the surfacing layer were improved under longitudinal magnetic field. But magnetic field frequency has an optimal value; otherwise grain refining and the mechanical property can’t achieve the best effect.