When pseudostatic method is employed to analyze slope seismic stability of high earth- rock dams, earthquake load is computed according to dynamic seismic coefficient figure in the present Specifications for Seismic Design of Hydraulic Structures (DL5073-1997). The figure is only suitable for earth-rock dams lower than 150m. While at the present time, many earth-rock dams to be constructed are higher than 150m. Compared with low dams, high order self-vibrating period of high dams is easy to coincide with seismic predominant period, and high order vibrations are constantly to be activated and amplified, which result in seismic acceleration distribution is different from low dams. With analytical and finite element method respectively, seismic acceleration distribution of high earth-rock dams is analyzed. Suggested dynamic seismic coefficient figure of earth-rock dams 300m-level in height is put forward, which offer a supplement for the present Code for Seismic Design of Hydraulic Structure.