Abstract: Magnesium alloy plays an important role in weight-loss because of its high specific strength, most of magnesium alloy components are casting. The defects of magnesium alloy castings become a manufacturing bottleneck issue. The repair process of magnesium alloy castings by tungsten inert gas (TIG) welding was studied, the effects of protect gas on the form, microstructure and properties of TIG repair beads were examined. The effects of helium content on the penetrations of welding beads were analyzed through the arc shape and arc voltage of TIG welding under different helium content. The results showed that the form of the weld bead could be improved by changing the helium content, the penetration and the penetration to width ratio could be improved by increasing the helium content, with no obvious effect on the microstructure and the hardness. The results of the simulating defect repair experiments showed that, the He-Ar mixed gas TIG welding repair process could repair shallow defects of magnesium alloy castings, increasing the helium content in the protect gas could increase the penetration and the suitability of this repair process.