Abstract: Trace CO₂ was added in the shielding gas during MGH956 alloy arc-ultrasonic TIG welding, and the mechanism and influence law of trace CO₂ on the microstructure and properties of the joint were analyzed. The results show that the grains in the weld grew smaller, the porosity in the weld became less, and the tensile strength of the joint increased. Based on the effect of the arc-ultrasonic, after 0.5% CO₂ was added in pure Ar, the grains in the weld center were fine-equiaxed and uniformly distributed in the weld, the tensile strength of the joint enhanced significantly, and the joint fracture mode changed from brittle fracture to ductile fracture. Meanwhile, the hardness of the weld increased after trace CO₂ was added in the shielding gas, and the comprehensive properties of the joint were improved.