In order to investigate the forming mechanism of local amplification in rail traffic induced ground vibration attenuation, the ground was simplified to be a single soil layer overlaid on an elastic half-space. Dynamic Green's function of vertical ground displacement of the three-dimensional layered site model was derived in frequency domain. The results show that local amplification is controlled by an important buried velocity interface, the location and intensity of the amplification depends on the ratio of elastic modulus of the media in the both sides of the interface (i. e. impedance ratio), the thickness of the overburden layer, exciting frequency and damping ratio of the overburden layer, from analysis of influence on the amplification of factors as modulus ratio of half space to overburden layer media with seven values, overburden layer thickness with four values, exciting frequency with seven values, overburden layer damping ratio with three values. In general, the larger impedance at the interface, the more obvious amplifying; keeping the other conditions the same, the closer of directly arriving time of surface wave to the arriving time of refraction wave from the interface governed by the overburden layer thickness, the more obvious amplifying; the closer of the peaks of the waves governed also by the exciting frequency, the more obvious amplifying; the smaller damping ratio of the overburden layer, the more obvious amplifying.