Abstract: As reinforceing,the Houcun slope in Qingshui County,Shanxi Province, I~III segment projects was worked well while the restaining wall in the IV segment was broken down.Using strength reduction FEM,the calculated C,φ value is 1.29 times less than the tested value.In the reduction,a new slide surface is produced on the concentration of shearing stress and this new slide surface is calculated as a new shear outlets.The thrusting force is compared between the new slide surface and the old one.It is calculated that the restaining wall is broken because of the new thrusting force.The calculation results is consistent with the real condition.