The dynamic shear modulus of loess under bidirectional dynamic loading is determined by using a SDT-20 dynamic triaxixal apparatus. The experimental results show that although the initial cyclic deviatoric stress and radial vibration amplitude trivially influence the Gd-γd (dynamic shear modulus-dynamic shear strain) curves of loess, they can significantly affect Gd-N (dynamic shear modulus-cycle number) curves of loess. The dynamic shear modulus at the identical cycles decreases with the increase of initial cyclic deviator stress and radial vibration amplitudes. At the same dynamic shear strain, the dynamic shear modulus of loess increases significantly with the growth of consolidation ratio. There exists a threshold of cyclic deviator stress under bidirectional dynamic loading condition, which is about 0 kPa. When cyclic deviator stress is lower than the threshold, the dynamic shear modulus of loess increases with the growth of dynamic shear strain, but when cyclic deviator stress is higher than the threshold, the dynamic shear modulus decreases with the growth of dynamic shear strain. By reducing the shear modulus at the inflection point and with the modified Hardin-Drnevich model, the Gd-γd relationship of loess under the bidirectional dynamic loading condition is quantitatively described, and the applicability of the proposed model is demonstrated.