Based on practical data obtained in microzonation of Baoji City and Xi'an City, a random 1-D model for soil layer is set up in this paper. Using the theory of random vibration and the method of perturbation, the effect of random site condition on the earthquake response analysis of soil layer has been studied preliminarily. The results show that the randomness of soil mechanical parameters usually can make the variance of earthquake response on the surface of soil layer has a change of 20% around its mean value, and it also can make the free frequency of soil layer has a change between 15%～40%. So that, it should be paid more attention to the randomness of soil mechanical parameters on the earthquake response analysis of soil layer.