Laser beam welding is used to fabricate the 7A52 aluminum alloy plates. Effects of shielding gas and defocusing on porosities are investigated. Porosities are divided into hydrogen porosities and keyhole-induced porosities. With the increasing flow rate of the front shielding gas, the porosity ratio firstly decreases, then increases. The porosity ratio is lowest under the flow rate of 25 L/min. When the flow rate is 30 L/min, the porosity ratio is higher because the large flow rate can affect the stability of the keyhole. The porosity ratio is also high-er when the flow rate is less than 25 L/min because the protection is weakened. With the increase of the defocusing, the porosity ratio firstly decreases, then increases. When the defocusing is –2 mm, the porosity ratio is lowest. When the defocusing is more than 0 mm or less than–4 mm, the porosity ratio is higher due to the movement of the instable keyhole.