Magnesium alloy low-pressure casting method

The invention is the magnesium alloy low-pressure casting method, which is to put the protective gas creator on the cover of the crucible or in the crucible that is in the pressure tank of the casting machine. The protective gas creator comprises: the protective agent 1, 2 and 3; the protective 1 is composed of sulfur powder and boric acid powder or of the mixing of sulfur powder, boric acid and magnesium sands; the protective agent 2 comprises sulfur powder or pyrite and carbon powder; the protective agent 3 is sulfur powder or pyrite, or the mixture of sulfur powder, pyrite and boric acid. The side wall of the container filled with the protective agent 1 has gas outlet and in the crucible; the protective agents 1 and 2 are in the open containers and on the crucible cover. The invention ensures the safe operation of the low-pressure casting, improves the quality of magnesium alloy cast, lowers the cost and reduces the environmental pollution.