Abstract: AC A-TIG welding experiments of magnesium alloy were carried out to study the effects of surface activating fluxes on weld penetration.Elements including Te, Ti and Si, oxides including SiO₂, TiO₂ and V₂O₅, halides including MnCl₂, CdCl₂ and ZnF₂ were used as activating fluxes, respectively.It is found that, Te powder, ZnF₂ and CdCl₂ all can increase weld penetration dramatically.Especially for Te powder, weld penetration reaches 1.6 times of that of conventional TIG welding.Weld depth width ratio reaches 0.43.Ti powder has little effect on weld penetration and weld width.All V₂O₅, SiO₂, TiO₂, MnCl₂ and Si powder decease weld penetration and weld width.Among the three activating fluxes improving weld penetration obviously, both Te powder and ZnF₂ refine weld grain.CdCl₂ coarsens weld grain a little.The results indicate that, electron adsorption with activating flux particles of Te powder, ZnF₂ and CdCl₂ can constrict arc and increase weld penetration of AC TIG welding of magnesium alloy.