Provided is a mixed active agent for activing flux tungsten inert gas arc welding of a magnesium alloy. A method comprises the following steps that (1), the mixed active agent is formed by mixing MnCl2 with ZnO, wherein the mixed active agent comprises, by weight percentage, 65%-75% of MnCl2 and 25%-35% of ZnO; (2), according to a use method of the active agent, MnCl2 and ZnO active agent powder is fully ground, after even mixing, an appropriate amount of acetone is evenly added, and the mixture is made into a paste; (3), the upper surface of a sample is evenly coated with the pasty active agent with a flat hairbrush, the coating width is about 40 mm, and the average coating amount of the active agent is 6 mg.cm<-2>; (4), TIG is conducted after acetone is volatilized; and (5), after welding is completed, the morphology and the microstructure of a welding joint and a molten bath are observed, and representation of the mechanical property is conducted.