Abstract: The effect of Ar and He dual mixed shielding gas and Ar, He and CO₂ ternary mixed shielding gas on the number of the gas pore and microstructure in the weld was studied in arc welding of 2519 aluminum alloy.Results indicate that, compared with using Ar alone, the number and the size of gas pore is reduced by using Ar andHe dualmixed shielding gas and the dual mixed shielding gas is also helpful for the transition of columnar crystal to equiaxed crystal.When the percentage of He reaches 70%, the number and the size of the gas pore are reduced significatly, and in weld centre it can get the smallest equiaxed crystal entirely.When filling 1% CO₂ into the dual mixed gas which the ratio of Ar:He is 30:69, the number of the gas pore is further reduced, but there is no evidence to prove that it can affect the microstructure of the weld. The width and the softening extent of the HAZ can also be reduced by filling He into Ar shielding gas.