Abstract: To solve the porosity defects in aluminum alloy joints obtained by laser-MIG hybrid deep-penetration welding, this article presents a novel laser-MIG hybrid welding method through applying ultrasonic vibration to the work piece. The number, size and distribution position of porosities in building-up layer on A7N01 aluminum alloy are investigated by X-ray detection and cross-section observation in stereo microscope. The effects of ultrasonic vibration on the morphology, microstructure and mechanical properties of the joints are also studied. The results show that the number and size of the porosities in building-up layer by laser-MIG hybrid welding significantly decrease, due to ultrasonic vibration accelerating the gathering and rising of small size porosities. The belt of columnar grains along fusion line near the building-up layer becomes thinner with the effects of ultrasonic vibration. The impact toughness and tensile strength are improved in some extent due to the introduced ultrasonic vibration in the laser-MIG hybrid welding method, so that it is supposed to be a good way for aluminum alloy welding.