Abstract: The reasonableness and limitation of the Seed’s method of the significant cyclic number for analyzing the large deformation of soils and differential settlements of subsoil during earthquakes are discussed on the basis of dynamic biaxial tests and the post - earthquake investigation. The results indicate that the significant cyclic number method is not suitable for analyzing the residual strain of soils under the seismic loads with the large pulse. Also, the effects of the asymmetry and irregularity of seismic ground motion, the anisotropic property of soil and transverse non-uniform distribution of soil layer on the permanent deformation of soils and earthquake-induced differential settlements of subsoil can not be considered by this method. Some unreasonable results will be obtained when this method is used to calculate the time history of the deformation of soils under earthquake loading and the earthquake-induced differential settlements of buildings.