In this paper, using Chinese aseismic code (TJLL-78) and statistical information about earthquake damage of multi-story brick building, according to the structure mechanics method, we get formula about assessment PGA. Having investigated, the damage of multi-story brick building of Jan. 4, 1988 Lingwu earthquake, Ningxia, we give PGA and intensity of this earthquake by calculating the coefficient of wall aseismic strength.