Abstract: A new nonlinear dynamic constitutive model of the loess is presented based on the arc-tangent function. In order to get the loess dynamic stress-strain curve and the theoretical curve of the proposed constitutive model of the loess, dynamic triaxial tests of Xi'an loess are conducted. Comparison between the theoretical and experimental results indicates that, compared with the H-D model, the new dynamic constitutive mode proposed in this paper is more suitable to describe the strength and deformation characteristics of Xi'an loess under the triaxial stress condition. The results obtained in this paper show that, with the same loading frequency and moisture content, the dynamic shear modulus of Xi'an loess increases with decrease of the moisture content. With the same confining pressure and loading frequency, dynamic shear modulus of Xi'an loess is increasing with increase of the confining pressure. With the same confining pressure and moisture content, dynamic shear modulus of Xi'an loess increases with the increase of loading frequency.