In this paper, the horizontally stratified soil layers are simplified as a shearing type multi-degree of freedom linear system. The transfer function of the system is determined in accordance with the statistical analysis of the result from the mode superposition method of soil layer earthquake response. By referring to the method of finding the extreme value in probabilistic theory, a response spectrum corresponding to the power spectral density of soil layers is given. At the same time, distributions of maximum response values of acceleration, relative displacement, shearing strain and Clearing stress of the soil layers along the depth are given according to the mode root mean square procedure. Eesults agree well with that of non-simplified methods.