The invention relates to an annealing isolation agent used for preparing mirror surface-oriented silicon steel with an excellent magnetic property. The annealing isolation agent comprises the following components by weight: 77-98% of Al2O3 powder, 1-8% of alkaline earth oxide powder, and 1-15% of an alkali chloride or/and an alkaline earth metal chloride. With the annealing isolation agent of theinvention, a glass membrane underlayer cannot form on a steel plate surface during high temperature annealing. Meanwhile, by means of corrosion reaction of a chloride, an embedded oxide in a substrate near-surface can be removed, so that a product with a smooth and clean surface as well as a stable magnetic property can be obtained.