Magnesium-based alloys are finding extensive applications foreground in aerospace and automotive applications. Weldability of magnesium alloys has recently been investigated with a variety of processes. In this article, the activating flux TIG (ATIG) welding of magnesium alloys with three single-component fluxes (TiO2, Cr2O3 and SiO2) under alternating current (AC) mode was studied. The effects of welding speed, weld current and electrode gap on the weld shape and the weld arc voltage in AC TIG welding with oxide fluxes were investigated on an AZ31B magnesium alloy substrate. The mechanisms of oxide fluxes on the arc shape and the arc voltage on the weld shape are discussed. The result showed that the TiO2 and Cr2O3 increase the weld penetration of AC TIG welding of magnesium with good bead cosmetics. The SiO2 increased the weld penetration with very poor formation of the weld surface. However, the arc voltage decreased with the used of TiO2 flux, and increased with the used of Cr2O3 flux. The mechanism of TiO2 and Cr2O3 fluxes increasing penetration should not accord with the "arc constriction". It would comply with some potential effects of the flux interacting with the liquid metal of fusion zone.