The operating principle of and the calculating method of the key technology parameters of precise system for mixing the protecting gas of magnesium-Alloy smelting are described. This system adopts the "static-pressure mixing" technology, which has two gas vessels to provide the gas in turn. This technology, compared to the current "dynamic throttling commingle " technology, enjoys higher precision of mixing and stability of gas-providing. The economic precision of the mixing is 0.1 percent, which can obviously reduce of the commingling-ratio density of the valid elemental gas' component and the operating cost of the equipment on the basis of ensuring the safety of segregation the oxygen during the magnesium-alloy smelting.