Abstract: The declining attenuation coefficients of the standard response spectrum index γis analyzed using the three-dimensional finite element method. The impact of index γon high earth-rock dams is analyzed based on the results proposed by Abrahamson and Silva (2007) whose research is funded by the American project “Next Generation Attenuation” (NGA). The suitable value for the index γis then discussed. The results show that the declining attenuation coefficient γof the standard response spectrum taking 0.7 is more suitable than 0.9 for the earth-rock dam which is higher than 200 m.