The silicon of magnesium ingot comes from crude magnesium, which exists in the inner and outer surface layer, can be elemental silicon, silicon oxide (SiO2) or magnesium silicate (Mg2Si), mainly in the form of Mg2Si followed by SiO2. When the vacuum of reduction is less than 15Pa, magnesium silicide is produced, and the dust on the surface of pellets is large, but the pellet formation rate is not high, and the powder is too much. When the pumping rate is high, the dust is brought to the crude magnesium in the mold. The insulation baffle is not installed or is not designed properly can result in high silicon. In the later stage of reduction, the crystallizer can not be quickly taken out of the tank, and the temperature rises sharply when magnesium is burned, and other compounds of silicon can also be born. Finally, the refining temperature is not good, the use of improper flux, the content of silicon can not be reduced.