Abstract: In the case of soft ground or large exciting force, the soil in the certain depth of the foundaon will assume the elastoplastic property, which results in the decrease of the reonant frequency of the nonlinear vibation of foundation In this paper, three simplified methods for the nonlinear resonant frequency of the fpimdation are reviewed and compared with experimental data .A polynomial interpolation of the relationship between reaction force and displacement of subsoil is proposed, which leads to simpler governing equation for the foundation vibration (Duffing equation and more accurate calculation of the nonlinear resonant frequency .