Abstract: DCSP (direct current straight polarity)A-TIG (activating-tungsten inert gas) 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, and oxides including SiO₂, TiO₂ and V₂O₅, and halides including MnCl₂, CdCl₂ and ZnF₂ were used as activating fluxes, respectively.It is found, the weld penetration and width of Te powder reach 2.5 times and 1. 4 times of those of conventional TIG welding, respectively.Ti and Si have small effects.Both V₂O₅ and SiO₂ increase weld penetration and width.Especially for SiO₂, the weld penetration and width reach 2.0 and 1.6 times of those of conventional TIG welding.TiO₂ has small effect.All ZnF₂, CdCl₂ and MnCl₂ increase weld penetration and width obviously.The effect of ZnF₂ is the most dramatic.The weld penetration and width reach 4.0 and 1.6 times of those of conventional TIG welding.It is believed that, the mechanism of activating fluxes improving weld penetration of DCSP A-TIG welding of magnesium alloy is relative to heat input increase because of activating fluxes constricting arc.