The invention discloses a method for preparing an AGM separator with high specific surface area. The method comprises the following steps: preparing glass fiber slurry from water and glass fibers, adding water for diluting, and regulating the pH value; performing negative pressure suction filtration and dehydration so as to obtain a primary molded wet separator; and applying nanosized silica aqueous dispersion to the surface of the wet separator, performing negative pressure suction filtration and dehydration so as to obtain a secondary molded wet separator, and drying, thereby obtaining the AGM separator with high specific surface area. The AGM separator with high specific surface area disclosed by the invention can achieve the effects of improving the stability and uniformity of a colloidal network structure and improving the layering phenomenon of electrolyte; meanwhile, the problem that the AGM separator is low in wet elasticity and low in assembly pressure retaining capacity is effectively solved, the pore size of the separator can be greatly reduced, the performance of preventing short circuit caused by dendrite puncture can be further improved, capacity degradation of a storage battery is retarded, and the cycle service life of the storage battery is prolonged.