Based on the relationship between power spectral density function and response spectrum, an expression describing an uniform modulation process input power spectral density function was established. The response spectrum method for the uniform modulation process was proposed for non － stationary response of soil － structure system under seismic excitation. The new code acceleration response spectrum was used as the object spectrum, corresponding power spectral density function used as inputspectral density function and the random seismic response of soil － structure interaction system was analyzed. The research result showed that the proposed method can describe the dynamic characteristics of earthquake motions and interaction system, and is an effective approach for soil － structure interaction system analysis. 