According to measured shear wave velocity of Xi’an Bell Tower area (Loess Area), the dynamic parameters of site soil are determined by using the relationship between shear wave velocity and compression wave velocity. Using Matlab program, the finite element size for low frequency subway vibration is obtained by analyzing soil dispersion phenomenon. On this basis, two-dimensional model with viscous - elastic boundaries is established by using the ANSYS program. The load-time history of the train is applied to the right tunnel, and the effects of the depth and breadth of the different models on the ground vibration velocity are discussed. Finally, the dimensions and element sizes of finite element model are obtained for the Xi'an No. 2 Metro Line with 15m depth in the loess regions.