The dam of Rimian Hydropower Station is a core wall type rockfill dam with height of 346 m. The situation of seepage field in the dam ,and especially in the core wall, is very important to the dam safety. The seepage field under different conditions was simulated with finite element method. In comparison with the simulated results, it is concluded as follows： The dam zoning and the seepage field distribution are reasonable while the filter layer Ⅰ is more permeable than the dam core with 1 magnitude and the ratio of the core wall slope is 1∶0.25. However, the safety of the core wall would much more rely on the protection by filter layer if the permeability difference is less and the core wall slopes are steeper. The transient seepage field is simulated under the condition that the reservoir water level falls in 5 days with speed of 8 m/d. The results indicate that the phreatic surface in the upstream dam shell falls almost synchronously with the reservoir water level, and the small hysteresis would not worsen greatly the seepage and the slope stability. The research results provide important references to the design of the dam at the pre-feasibility study stage.