Abstract: After a reservoir was filled with water, the effect on statical deformation in the rock medium of it's foundation is caused by the load of water mass. Morever, the deformation also results from the dilatancy in the rock medium at the foci when some earthquakes occur. In this paper, these two types of deformations have been considered.The total effect of the load of the water mass on statical deformation consists of the statical elastic deformation in rock medium resulting from the water load, the deformation of the gravitational equipotential surface caused by the water mass, and that resulting from elastic deformation. In addition, the upheaval of ground surface due to dilatancy in the rock medium occurs also at certain stages. It induces, at the same time, the variation of the Earth's gravitational field.The results obtained from first order precise levelling prior to and after the filling of the Danjiang reservoir have been analysed based on the model mentioned above. Lame elastic constant of rock medium has been determined and the relation between vertical deformation of ground surface and the earthquakes occurred in this area has been discussed. It is supposed that with this model, studies on the characteristics of the vertical deformation of ground surface after the filling of the reservoir, then, may be possible to acquire certain information for earthquake predication in the area.