Abstract: According to the effect of water infiltrating in porous rocks and the generalized classification of earthquake by Bell and Nur and Mogi,it is suggested that the process of infiltrating of water may be responsible for the unstable and inhomogeneous distributioneh of the effective stress over the reservoir area. The characteristics of the earthquake sequence in the reservoir are related to this kind of stress distribution. The effect of infiltrating water on the main shock is discussed. The permeability can be found to be 10^-4 darcy. It is suggested that the tectonic stress in the reservoir area is not in the critical state; water in the reservoir area collected along with underground water to form a new water system; and then water widely infiltrated into deeper parts with the pore pressure increasing and the tectonic stress concentrating to cause the main shock. On the basis of the Coulomb criterion,the transformation of the stress field is studied. The transformation of the stress field after the main shock may account for different stress state of earthquake sources between the foreshock and the aftershock.