Abstract: The scattering from the external ring-stiffened finite cylindrical shell has been studied before and the result shows the secondary scattering has important contribution to the scattering field.The primary Planar Elements Method (PEM)cannot accurately forecast the echo characteristics.In this paper,based on the primary PEM,a modified PEM is developed by adding the judgments of occlusion and secondary scattering and combining with the bistatic PEM.The Target Strength of the dihedral reflector is calculated both by the primary and modified PEM and then the validity of the modified PEM is shown by comparing with the theoretical solution.Then,the Target Strength of the external ring-stiffened finite cylindrical shell is also calculated.The result shows that occlusion and secondary scattering respectively have important contribution to the scattering field at different incident angle.When considering the transmission of the rings,the secondary scattering is strong in high-frequency but weak in low-frequency.Finally,the good agreement is shown between the numerical result and the experimental result.