In order to study the mechanism of water damage of an asphalt pavement, the FLAC3D program was adopted to model and analyze the mechanical response of a saturated asphalt pavement under instantaneous vehicle load. The results show that the horizontal stress, vertical stress and shear stress of an asphalt concrete pavement increase with the increase of instantaneous load. The surfaces of asphalt pavement structural layers are most vulnerable to damage. The horizontal stress, vertical stress and shear stress decrease sharply with the instantaneous dynamic load decreasing to zero. The horizontal stress reaches maximum value at the interface between the base and the large stone porous mixture (LSPM) layer, while the maximum vertical and shear stresses occur on the surface layer of the saturated asphalt pavement. The deformation decreases almost linearly from the surface of the asphalt pavement to the subgrade, and the pore water pressure was little influenced by the transient load.