Problem exists in the tradition studies of the stratified soil slope back analysis. The limitation of the tradition method was analyzed on the basis of previous research in this paper. And the definition of the soil layer displacement, which overcomes the size effect of the soil layer, was presented. This displacement was weighted in the objective function, reflecting the sliding characteristics of different soil. Then the calculation method weight of weighted was presented. At last, the strength parameter sensitivity of the objective function was calculated by an example. The result showed the stratified soil slope back analysis can be solved by the new method.