The influences of the He-Ar mixed shielding gas on the droplet transition behaviors and the weld porosity defects are investigated in the laser-MIG hybrid welding of aluminum alloys. According to the dynamic characteristics of the keyholes in the welding process, the reason for the suppression of porosity defects is analyzed. The results show that, the He-Ar mixed shielding gas can enhance the stability of the keyholes and reduce the weld porosity effectively. When the volume fraction of He in the He-Ar mixed shielding gas is 50%, the droplet transition mode changes from “one drop per pulse” to “short-circuit” transition and the weld porosity is about 1.0%, reduced by 80% if compared with that for the pure argon shielding gas. The further increase of the He content is not helpful to the suppression of the porosity. When the shielding gas is the pure He gas, the big spatter and the bad welding seam occur.