Defect of the porosity always occurs during laser welding of 5754 aluminum alloy and has a lot of bad influence on its mechanical property. Effects of the gap between two lap welding sheets on the porosity have been studied using fiber laser to weld 5754 aluminum alloy. It is found that setting a proper gap between two sheets can provide a passage for the escape of the porosity and decrease the porosity rate. When the gap changes from 0 to 0.2 mm, because of the capillary phenomenon, the escaping passage is closed by the solidification of the liquid weld metal, and the porosity decreases little. When the gap changes from 0.30 mm to 0.75 mm, the capillary phenomenon of the liquid weld metal between the gap weakens. So this part of weld metal is close to the fusion line and keeps liquid state. Then the escaping passage is opened and the porosity decreases obviously. It is also found that as the gap increases, the shear stress increases a lot.