The shear dilatancy is one of the important features of soil.However,it has been overestimated by the classic plastic mechanics,thereby causing the deviation of soil strength from practice.We modify the shear strength parameters of Mohr-Coulomb criterion by the using of equivalent internal friction angle,which reflects the influence of dilatancy on soil strength.Based on the failure mechanism with a logarithmic spiral shape,we use the equations to calculate the internal dissipation,derive external work of slopes,and establish a method to evaluate the safety factor of slop by the theories of limit analysis and of strength reduction.The results show that the results computed by numerical modeling test are reasonable.The safety factor increases more slowly with the increasing of the dilatancy angle,and the trend is non-linear.The recommendations of the values of dilatancy angle are given in practice work.