Abstract: A prediction-method of large ground displacements caused by seismic liquefaction is presented in this paper.The formulae of displacement during earthquake, large ground displacement of non-liquefaction layer which is laid on the liquefaction one induced by duration of pore water pressure diffusion after earthquake, displacement and veloaty of liquefaction materials are deduced on the principle of variation and the method of fluid mechanics. In order to check these formulae, take the example of the large ground displacements caused by Shibiyuan sandy loess seismic liquefaction which was triggered by 1920 Haiyuan earthquake(Ms=8.5),Ningxia Autonomous Region of Hui-Minority, China. The computed results quite agree with the evidences in field.