Abstract: Some adverse effects may bring forth to safety of the core wall rockfill dam when it is impounded for the first time, especially when the impounding speed is too fast. For example, there may be settlement increases in later stage, arching effect enhancements even cracks of the core wall, seepage deformations, instability of downstream dam slope and soaking deformations of upstream rockfill. So it is necessary to study the relation between impounding speed and the safety of dam. Based on the experience of Nuozadu core wall rockfill dam, by means of numerical calculations and analysis, in aspects of deformation, stress, hydraulic fracture, unsteady seepage and dam slope stability, safety characteristics of the dam when impounded for the first time was studied and the impounding speed was advised.