Abstract: Narrow gap welding has a greater advantage in the thick plate welding,the protective gas composition has the influence on the arc behavior,droplet transfer and the weld formation. Aiming at narrow gap GMAW of 30 mm thick Q235 steel,the effect of different shielding gas compositions on the weld formation was studied in the ternary shield gas composed of Ar,He and CO₂. The weld is well formed when the content of He is set at 5% and the content of CO₂ is up to 10%,and the weld with smaller finger penetration and larger sidewall penetration can be obtained. The weld appearance is good at the CO₂ content of 10% and the He content of 10%,and the finger penetration is smaller and the sidewall penetration is larger.