Abstract: Triaxial compressive experiments of porous rock samples were carried out under various confining pressure and initial pore pressure without drainage; axial strain and pore pressure were observed versus differential stress. The results of such experiments show that pore pressure increases with increase of differential stress at low differential stress; pore pressure decreases with increase of differential stress at media and high differential stress. Pore pressure also increases with large amplitude decrease of differential stress at high differential stress. Based on such experiments, it is suggested that water table anomaly before earthquakes reflects the change of differential stress in crustal rocks. The anomalous behavior of water tables in the epicentral and peripheral areas before the great Tangshan earthquake of July 28, 1976 are explained by such suggestion.