Abstract: A treatment method for a magnesium slag, comprises: Step a, producing magnesium particles and a crude solution of magnesium slag by digesting and sifting a magnesium slag; Step b, filtering the crude solution of magnesium slag sifted in Step a, so that mixed chlorides are obtained after a moisture in a filtrate is removed; Step c, obtaining a high purity magnesium oxide by dissolving a filter residue obtained in Step b via an ammonium sulfate method and a magnesium precipitation reaction as well as post-treatment. With the method, utilization of magnesium slag can reach up to more than 90% with a higher recycling rate, while the discharge of solid wastes can be reduced greatly which solid wastes are less contaminative to the environment, so that the contamination to the environment is greatly reduced and the required energy saving and emission reduction are also achieved.