Abstract：Based on the Yu Maohong's unified strength theory，a new method is proposed to study dynamic strength parameters of geomaterials and strength theory under the complex stress state. Considering the property of repeated loads and the effect of intermediate principal stress，the concept of dynamic limit equilibrium under unified strength is put forward by coupling the dynamic strain with the theory of limit equilibrium，and then the equations that can determine the effective strength index and are obtained. A simple method is also advanced to determine the effective strength index and which takes account of the concept of deviator stress，effective consolidation pressure and the dynamic equilibrium line of unified strength theory. This method can be applied to sandy soils，silty soils and deposit soft rock.
