Abstract: In the paper, the feature of surface ground motion on the liquefied sites is presented by theoretical calculation and analysis with earthquake data. The results of the calculated and the observed data are agreeable qualitatively and some important results are consistent quantitatively. Both indicate that the liquefaction makes the frequency of the surface horizontal quake reduced obviously. Also, the liquefaction makes the characteristic period of the response spectra of the surface horizontal accelerations prolonged by more than 0.1 s at least and 0.3 s on average. The period of 0.8 s in the response spectra of the surface horizontal accelerations is a boundary point, below which the acceleration of liquefied soil layer decreases in the spectra to a certain extent and beyond which the acceleration of the liquefied soil layer significantly amplifies,especially in the range of 1 s～3 s in the spectra.