This paper simplified the model and the equilibrium equations on the composite thin-walled beams. According to the boundary conditions of a cantilever beam, natural frequencies of box and circular beams in the directions of lead-lag, flapping and twisting were contrasted with those in a related reference to verify the validity of the model. An equivalent uniform solid beam whose length, cross section shape and line density were the same with those on the composite thin-walled beam was also built. By contracting and analyzing the natural frequencies of two beams, the orthogonal anisotropic effective elastic modulus expressions of composite thin-walled beams in the directions of x, y, z and twisting can be obtained. The approximate effective moduli on box and circular beams were calculated under the CUS, CAS configuration and other special layer styles. The effect of ply angel, ply thickness, the length, layer style and cross section on the effective moduli was also discussed. Finally, two calculating examples were furnished to demonstrate that much dynamic analysis on the composie beams can be made by the classic beam theory using an approximate effective modulus method
