Abstract: The processes of reservoir impoundment and emptying have important impact on seepage safety of earth-rock dams. The seepage control is one of the key technical problems for 300 m-level super-high core wall dams. To investigate the effect of change rate of water level on the seepage stability of dams, a three-dimensional (3D) finite element model for a 300 m-level gravel-soil core wall rock-fill dam is established. The seepage processes of reservoir impoundment and emptying are simulated, and the change of seepage surface and seepage gradient of core wall are analyzed. The simulated results show that the seepage surface has a steep drop in the core wall and continues to develop from the upstream to the downstream when the water level rises during the impoundment. The faster the water level rises, the steeper the seepage surface in the core wall. When the reservoir is be emptying, the seepage surface in the core wall lags behind with the reservoir water level, and the free surface has convex shape. The faster the water level drops, the greater the difference of the convex seepage surface forms in the core wall.