The reduction theory for elastic modulus and Poisson’s ratio is given according to the stress-strain hyperbolic curve of the soil. The result of an example shows that the Poisson’s ratio has a significant influence on the safety factor and the plastic zone of the slope; but the elastic modulus don’t have a significant influence; the displacement controlled-method is suggested to take for the finite element analysis to solve the safety factor of slopes because of this method is good in stability and convergence.