The calculation expressions of seismic active earth pressure and modified earth pressure coefficient are derived based on the basic hypothesis of Coulomb’s earth pressure theory and a pseudo dynamic method. The formulas focus on a rigid retaining wall with cohesionless backfill and consider two kinds of conditions with vertical steady seepage. After the problem is solved by computer programs and parametric discussion is taken

the results indicate that the active earth pressure increases significantly with the increase of horizontal seismic acceleration; the effect of vertical seismic acceleration on earth pressure is relatively small

which can be ignored. The earth pressure decreases monotonously with the increase of soil friction angle when soil-wall friction angle is small; however

increasing soil friction angle makes the earth pressure initially decrease and then increase as soil-wall friction angle becomes larger. The seepage effect is also investigated

it is shown that the active earth pressure decreases with the increase of hydraulic gradient for downward seepage case

but increases for upward seepage case instead. Through comparison

the calculation results agree well with the existing methods so as to verify the validity of the proposed method.