Abstract: The oscillating laser-arc welding was carried out on the 6mm thick 6061 aluminum alloy locked bottom butt joint commonly used for the roof of high-speed rail, and the effect of oscillating frequency on the microstructure and mechanical properties of the weld was systematically studied based on SEM, EBSD and other characterization methods together with the penetration and porosity of the weld as indicators. The results showed that the porosity of the joint was reduced to less than 1% when the oscillating frequency sharply increased to 250 Hz. The weld area near the fusion line was primarily arranged with columnar dendritic structures, while the central arc region of the weld was chiefly composed of equiaxed dendritic structures. Laser oscillating frequency could affect the beam advance distance in per unit cycle, which posed a significant impact on the stirring action of laser beam behind the molten pool. With the increase of the oscillating frequency, the stirring effect of the molten pool was enhanced, which effectively refined the central equiaxed structures and dropped the dendrite sizes from 68 μm to 44 μm. The tensile strength reached their maximum value at the oscillating frequency of 250 Hz, with a maximum tensile strength of 229 MPa, which was about 73% of that of the base metal. Both the reduction of the porosity and the refined dendrites structures of the weld largely created favorable conditions for the improved tensile strength.