Abstract: Based on the limit equilibrium theory and by treating the backfill as an ideal elasto-plastic material which followed the Mohr-Coulomb yield criterion,some drawbacks of the Coulomb’s earth pressure theory were pointed out,and it was explicitly postulated that the limit earth pressure resulted from the plastic soil body behind the retaining wall.By assuming that a family of slip lines in the plastic zone were straight lines,namely,the plane slip surfaces,a more reasonable slip wedge analysis model for clay was established to solve the passive earth pressure on the retaining wall,the reaction force on slip surface and their distributions.The classical Coulomb’s and Rankine’s passive earth pressures could be considered as the special cases of the solutions.