In this paper, it was verified that the dynamic constitutive relationship of unsaturated undisturbed loess under random seismic loading still obeies a hyperbolic model by means of a dynamic tri-axial test system. But the parameters of the model are different from different time histories of applied random seismic loading. The contrasting research was made in parameters, shear, modulus and damping ratio under different random seismic loading and equivalent sinusoidal loading. The quantitative relationship between the parameters of the dynamic constitutive model and predominant period of applied random seismic loading was initially established. Through tests and computation, it was confirmed that Mansing Law is suitable for loess.