Abstract: The motion equations of rotating composite thin-walled cantilever beams with variable closed-section are derived based on Lagrange’s equation. This dynamics modeling method is more concise than the one based on the Hamiltonian principle. And the dynamic model includes the warping effects of classical torsion-related warping and out-of-plane warping due to uniform axial extension and bending as well, but not includes transverse shear related warping. The simplified free vibration equations of two stiffness configuration taper rotating box beams are presented. And the corresponding solution is obtained by a Galerkin method. The finite element analysis software ANSYS is used to calculate the natural frequency of the rotating beams. And the results are compared with the approximated solutions obtained by Galerkin method. The composite elastic coupling, ply angle, taper ratio and rotating speed influences on the free vibration frequencies are investigated.