Based on the digital records of strong motion on deep soft soil sites and bed rock sites in the western U. S. A. , the attenuation laws of PGA on deep soil and bed rock are obtained respectively, and the amplifying effect of the soil is discussed. The results shows that the average amplifying effect of deep soft soil is not large. In detail the larger the PGA on bed rock is, the smaller the amplifying effect of deep soft soil is; the larger the epicentral distance is, the smaller the amplifying effect of deep soft soil is.