The present disclosure belongs to the technical field of casting, and specifically relates to a high-inertia mold shell for casting, a preparation method thereof, and a method for improving precision of a magnesium alloy cast. The high-inertia mold shell for casting includes a surface layer, which is configured to contact a magnesium alloy pouring liquid, wherein the surface layer contains an inorganic compound, and the inorganic compound is at least one selected from the group consisting of boric anhydride, borax, potassium fluoride, and sodium fluoride; and in the above, based on a total amount of raw materials of the surface layer, a content of the inorganic compound accounts for 10-40 wt %.