Abstract: To improve the quality of MIG welded joints of aluminum alloys, the ultrasonics were generated in the MIG welding arc with external ultrasonic excitation source, and the influence of the excitation frequency on the weld appearance, pore distribution, microstructure and mechanical properties of the 6061-T6 aluminum alloy butt weld was studied. The results show that arc-ultrasonic can effectively refine the microstructure of weld metal, reduce pores in the weld, and increase the strength of the welded joints. Compared with conventional MIG welding, arc-ultrasonic technique can increase the weld width, refine the cellular dendritic and enlarge the equiaxed grain zones in the weld metal. Microstructure refinement efficiency is most excellent when the excitation frequency is 30 kHz. As the excitation frequency increases, the number of pores in the weld gradually decreases, and the tensile strength of the joint is improved. The maximum tensile strength is 235 MPa, which is 73.4% of the base metal under the excitation frequency of 50 kHz, and accordingly the dimples in the tensile fracture have a more equiaxed shape and a more homogenized distribution.