Abstract: Activating tungsten inert-gas (A-TIG) welding with direct current straight polarity (DCSP) mode of aluminum alloy was used to study effect of surface activating flux on welding penetration and to fundamentally study the mechanism of increasing welding penetration. CaF₂,TiO₂, SiO₂ and AF305 flux for aluminum alloy studied by ourselves served as experimental fluxes. It was found that welding penetration varied with fluxes and was consistent with welding voltage. It is believed that heat input is one of important factors influencing welding penetration. Especially for SiO₂, the increase of heat input is the crucial factor because welding penetration varies synchronously with weld width.