An orthogonal experiment is designed to investigate the laser welding of a 30 mm thick 5083 aluminum alloy plate under different process parameters. The results show that, the laser power, the welding speed and the ambient pressure have different effects on weld formation and porosity, in which the effects of the ambient pressure on the porosity, the penetration depth and the depth-width ratio are the strongest. When the ambient pressure is decreased to 5 kPa, the penetration depth is twice that at normal pressure and the porosity is only 0.057%. This research result provides a reference for the control of weld formation and porosity in the laser welding of aluminum alloys.