Abstract: Three metal chlorides including KCl,MnCl₂ and ZnCl₂ were used as activating fluxes for the bead-on-plate ATIG welding on magnesium alloy,aluminum alloy and Q235 steel. From the viewpoint of first ionization energy of the element, the effect of the three fluxes on the penetration depth was studied,and the mechanism was also discussed. The results indicated that metal element in the fluxes would move into the arc plasma and had influence on the arc state in the A-TIG welding process. There is a close relationship between the weld penetration depth and the first ionization energy of metal element in the fluxes. The regularities of metal chlorides on different materials are the same,and with the increase of first ionization energy of flux metal element in the fluxes,the penetration depth increases firstly and then decreases.