Abstract: Saturated soil under a concentrated dynamic load in a tunnel will cause vibration displacements, which concerns the liquefaction, and shaked subsidence. The paper will be based on the tunnel’s section feature located in saturation soil, adopting a two-phase saturated medium’s Green function in BEM and Lamb integral equation to infer from Betti theorem to figure out the displacement function of the underside in the tunnel arose by vehicle-engine vibrations. Combined with the exact example, the value after being calculated indicates that the result is reliable.