Abstract: Based on the unified strength theories,taking a typical section of some highway roadbed slope as an example,and with Bishop's simplified method of limit equilibrium theories of rigid body,obtained the relationship of different parameters of unified strength theories,different parameters of twin-shear stress state and coefficient of the stability of slope.On the basis of research result,the effect of strength theories is discussed.By analysis,it can be concluded that the effect of intermediate principal stress in the stability of slope analysis shouldn't be neglected.In actual project,we should determine the parameters of unified strength theories and the parameters of twin-shear stress state according to the characteristics of geotechnical and the stress state of concrete slope,reflect the strength characteristics of geotechnical exactly,and limit the scope of stability coefficient,exert the potential of material effectively,thus,the project expense could be saved.