The complex structural dynamics model of large-span truss beam structures and the flexibility issues brought about by large spans make the analysis of system dynamics and vibration control relatively challenging. The establishment of an equivalent dynamic model is a key technology for conducting semi-physical simulation experiment platform system dynamic model analysis. A large-span flexible truss beam structure is designed from three aspects: the cross-sectional shape, cross-sectional uniformity, and cross-sectional connecting rods. For the large-span flexible truss girder structure, based on the principle of energy equivalence, the relationship between the kinetic energy and strain energy of periodic structures and the kinetic energy and strain energy of the equivalent continuous beam is established. The equivalent mass and stiffness matrices are solved based on the principle of equivalence between strain energy and kinetic energy and the theory of continuum mechanics to establish the equivalent dynamic model. The correctness of the theoretical model was verified through theoretical analysis of natural vibration frequencies and modes, and compared with finite element simulation results.