The truss element is described according to the structural composition characteristics of the ring-shaped supported truss，and the deformation assumptions are given. The equivalent polygon structure of the ring-shaped truss is obtained based on the beam model. The kinetic energy and strain energy of the equivalent polygon unit and its corresponding circumscribed circular ring unit are calculated. The relational expressions between the elasticity modulus and density of the equivalent polygon structure and the circumscribed circular ring are derived by using the energy equivalence principle. Thus，the expressions of the frequency ratio are obtained. The modal shapes and frequencies of the equivalent polygon structure and the corresponding ring structure are compared by using the finite element method and the theoretical method. The results show that it is reasonable to use the continuum ring model to simplify the equivalent polygon structure for the dynamic analysis of the ring truss structure when the number of elements is more than 30.