Simplified horizontal slice method is adopted to deduce the formula for the required tensile force of reinforcement
and the equation for the critical failure angle when the reinforcements are ductile materials. At the same time，the equations
have also considered both horizontal and vertical seismic force，both cohesion and internal friction of fill for reinforced
retaining wall，and groundwater et al.，and furthermore，the effect of these factors upon the critical failure angles，required
tensile force of reinforcement and coefficients for required tensile force of reinforcement is also considered. It is found that the critical failure angle increases with the increase of cohesion or internal friction of fill for reinforced retaining wall and reduces with the increase of horizontal seismic force，the influence of vertical seismic force versus critical failure angle and required tensile force of reinforcement is agreement with the results of Ling & Leshchinsky，the coefficients for required tensile force of reinforcement increase with the increase of groundwater table or pore water pressure ratio and reduce with the increase of internal friction of fill for reinforced retaining wall.